feat(setup): drop redundant Snowflake schema prompt; fall back to free-text on listSchemas failure

Snowflake setup previously asked for a single schema as free text, then
ran a multiselect against the discovered schemas — two schema questions
back-to-back, with the first being only a session bootstrap. The SDK's
`schema` is optional, so the bootstrap step is unnecessary.

- Remove the free-text Snowflake schema prompt; only pass `schema` to
  snowflake-sdk when one is configured.
- When `listSchemas()` fails (e.g. role lacks SHOW SCHEMAS), prompt the
  user for a comma-separated list, persist it as `schema_names`, and use
  it as both the table-list filter and the multiselect default. Applies
  to every driver with a scope-discovery spec, not just Snowflake.
- Update docs to lead with `schema_names`; keep `schema_name` as a
  documented single-schema shorthand.

docs-site/content/docs/integrations/primary-sources.mdx

@@ -129,20 +129,18 @@ connections:
     account: xy12345
     warehouse: ANALYTICS_WH
     database: PROD
-    schema_name: PUBLIC
+    schema_names:
+      - PUBLIC
+      - SALES
+      - MARKETING
     username: KTX_SERVICE
     password: env:SNOWFLAKE_PASSWORD
     role: ANALYST
 ```
 
-For multiple schemas:
-
-```yaml
-    schema_names:
-      - PUBLIC
-      - ANALYTICS
-      - STAGING
-```
+`ktx setup` discovers schemas after the connection is verified and writes the
+selected list to `schema_names`. You can also set this field manually. For a
+single schema, `schema_name: PUBLIC` is accepted as an equivalent shorthand.
 
 ### Authentication