fix(context): merge overlay columns onto manifest columns by name (#94)

* fix(context): merge overlay columns onto manifest columns by name

composeOverlay was appending overlay columns to the manifest column list,
producing duplicate entries when dbt/metabase overlays declared a column
just to attach descriptions. The duplicates carried no `type`, so the
pydantic SourceDefinition rejected them at semantic-query time and broke
`ktx sl query` for every overlay-backed measure. Now overlay columns
match base columns by name (case-insensitive): same-name entries merge
onto the manifest (overlay fields win, type/role fall back to the base,
descriptions merge per source key) and only new names append.

* refactor(sl): split overlay columns from column_overrides and enforce TS/Python wire contract

Overlay sources now have two distinct collections: `columns:` for computed
columns (requiring `expr` + `type`) and `column_overrides:` for metadata
patches to inherited manifest columns. Composing or loading an overlay that
mixes the two — or references an unknown column — fails with a typed error.

Introduce `ResolvedSemanticLayerSource` / `resolvedSourceSchema` /
`toResolvedWire` as the strict shape sent to the Python engine, and add a
schema contract test that diffs Zod against the Pydantic JSON schema dumped
by `python -m semantic_layer dump-schema`. `SourceDefinition` is now
`extra="forbid"` on the Python side.

`loadAllSources` surfaces per-file load errors instead of swallowing them,
so validation/query paths can report manifest shard parse failures.

* fix(context): make scan description generation resilient and quiet

A transient sampleTable failure during ingest used to take out every
table in a connection: generateTableDescription returned a hardcoded
'Table not found' string into descriptions.ai, and KtxDescriptionGenerator
was constructed without a logger, so the failure left no trail anywhere.

- sampleTable / sampleColumn calls retry 3x with 200/400/800ms backoff,
  honouring KtxScanContext.signal via a new KtxAbortedError.
- On retry exhaustion or missing capability, table generation falls back
  to a metadata-only prompt built from column name / native type / comment
  / rawDescriptions. The column path follows the same rule -- call the
  LLM when any of samples or rawDescriptions are available; skip only
  when both are absent.
- Logger is now threaded from KtxScanContext into the generator. Failures
  emit structured KtxScanWarning entries (new description_fallback_used
  code, plus existing sampling_failed / enrichment_failed /
  connector_capability_missing). ktx scan groups warnings by code so a
  batch of identical failures collapses to one summary line plus sample.
- Returns null on failure instead of the 'Table not found' sentinel; the
  manifest writer's existing guard already skips empty descriptions, so
  schema YAML no longer carries misleading text. SCAN_MANAGED_DESCRIPTION_KEYS
  already strips stale 'ai' on merge, so existing YAML clears on next run.

Also suppress AI SDK v6 'system in messages' warning: pull system messages
out of KtxMessageBuilder.wrapSimple's output via a new splitKtxSystemMessages
helper and pass them top-level to generateText (preserves cacheControl
providerOptions on the SystemModelMessage). Agent-runner's local
splitSystemPromptMessages dedupes onto the shared helper.

* test(docs): align examples-docs assertions with revamped docs

PR #103 (setup/guide doc revamp) reworded several CLI examples and
connection labels; the assertions in scripts/examples-docs.test.mjs
still referenced the pre-revamp wording and were failing in CI on main.
Update the regexes to match the post-revamp content:

- drop the `--json` flag from the sl-query example expectation
- move the `Driver:` / `Status: ok` probe to the connection reference,
  which is where that output now lives (driver id is lowercase
  `postgres`, not the display name `PostgreSQL`)
- drop the obsolete `Install \`uv\`...` troubleshooting line
- accept `<connectionId>` everywhere; the docs no longer use the
  hyphenated `<connection-id>` form
- match the `warehouse` connection id used in the quickstart instead of
  the `postgres-warehouse` id only used in the README and setup ref

* fix(sl): skip TS/Python schema contract test when uv is unavailable

The TypeScript checks CI job does not install uv or Python, so the
module-level `execFileSync('uv', ...)` in schemas.contract.test.ts threw
ENOENT and failed the suite. Wrap the schema dump in a try/catch and
guard the describe block with `describe.skipIf` so the test skips in
environments without uv. Local dev and any CI job that has uv on PATH
still runs the cross-language contract assertion.

packages/context/src/ingest/ingest-bundle.runner.test.ts

@@ -187,7 +187,10 @@ const makeDeps = () => {
     loadAllSources: vi
       .fn()
       .mockImplementation((connectionId: string) =>
-        Promise.resolve(connectionId === 'warehouse-2' ? [{ name: 'looker__orders' }] : []),
+        Promise.resolve({
+          sources: connectionId === 'warehouse-2' ? [{ name: 'looker__orders' }] : [],
+          loadErrors: [],
+        }),
       ),
   };
   const slSearchService = {
@@ -1347,7 +1350,7 @@ describe('IngestBundleRunner — Stages 1 → 7', () => {
       frontmatter: { sl_refs: ['looker__b2b__sales_pipeline.arr'] },
     });
     deps.semanticLayerService.loadAllSources.mockImplementation((connectionId: string) =>
-      Promise.resolve([{ name: `${connectionId}_source` }]),
+      Promise.resolve({ sources: [{ name: `${connectionId}_source` }], loadErrors: [] }),
     );
     deps.agentRunner.runLoop.mockImplementation(async (params: any) => {
       if (params.telemetryTags.operationName === 'ingest-bundle-wu') {
@@ -1447,7 +1450,7 @@ describe('IngestBundleRunner — Stages 1 → 7', () => {
       parseArtifacts: { semanticModels: [{ name: 'orders' }] },
     });
     deps.semanticLayerService.loadAllSources.mockImplementation((connectionId: string) =>
-      Promise.resolve([{ name: `${connectionId}_source` }]),
+      Promise.resolve({ sources: [{ name: `${connectionId}_source` }], loadErrors: [] }),
     );
     const postProcessor = {
       run: vi.fn().mockResolvedValue({
@@ -1631,7 +1634,10 @@ describe('IngestBundleRunner — Stages 1 → 7', () => {
     const deps = makeDeps();
     deps.adapter.listTargetConnectionIds = vi.fn().mockResolvedValue(['postgres-warehouse']);
     deps.semanticLayerService.loadAllSources.mockImplementation((connectionId: string) =>
-      Promise.resolve(connectionId === 'postgres-warehouse' ? [{ name: 'stg_accounts' }] : []),
+      Promise.resolve({
+        sources: connectionId === 'postgres-warehouse' ? [{ name: 'stg_accounts' }] : [],
+        loadErrors: [],
+      }),
     );
 
     const runner = buildRunner(deps);
@@ -1659,7 +1665,10 @@ describe('IngestBundleRunner — Stages 1 → 7', () => {
 
   it('does not resolve qualified fallback table refs by source name alone', async () => {
     const deps = makeDeps();
-    deps.semanticLayerService.loadAllSources.mockResolvedValue([{ name: 'orders', table: 'sales.orders' }]);
+    deps.semanticLayerService.loadAllSources.mockResolvedValue({
+      sources: [{ name: 'orders', table: 'sales.orders' }],
+      loadErrors: [],
+    });
     const runner = buildRunner(deps);
 
     await expect(