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.

python/ktx-sl/semantic_layer/models.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 from enum import Enum
 from typing import Any, Literal
 
-from pydantic import BaseModel, Field, model_validator
+from pydantic import BaseModel, ConfigDict, Field, model_validator
 
 
 # ── Source Definition Models ──────────────────────────────────────────
@@ -105,6 +105,8 @@ class DefaultTimeDimensionDbt(BaseModel):
 
 
 class SourceDefinition(BaseModel):
+    model_config = ConfigDict(extra="forbid")
+
     name: str
     description: str | None = None
     descriptions: dict[str, str] | None = None
@@ -123,6 +125,8 @@ class SourceDefinition(BaseModel):
     def validate_source(self) -> SourceDefinition:
         if self.description is None:
             self.description = _resolve_description_map(self.descriptions)
+        if not self.table and not self.sql:
+            raise ValueError("resolved source must have 'table' or 'sql'")
         if self.table and self.sql:
             raise ValueError("'table' and 'sql' are mutually exclusive")
         if not self.grain: