[codex] fix requestDimensions schema sizing and request payload alignment

[rwmjhb]

Supersedes #377.

What changed

• add explicit embedding.requestDimensions schema and UI metadata
• stop forwarding embedding.dimensions as a request-side override by default
• size the local/store vector schema from the same effective dimension logic used by the embedder
• add regression coverage for request-only overrides and omitDimensions

Why

PR #377 left the source of truth split in two places:

• request-time validation could use requestDimensions ?? dimensions
• LanceDB schema sizing still only used dimensions

That meant request-side overrides could pass embedder validation while the store schema was still sized from a different value. This PR makes the effective vector dimension calculation shared so local validation, store sizing, and request payload behavior stay aligned.

Impact

• embedding.dimensions remains the internal schema/validation dimension
• embedding.requestDimensions is now the explicit request-side override for providers that support variable output sizes
• omitDimensions now cleanly suppresses request-side dimension fields even when requestDimensions is configured

Validation

• node test/embedder-error-hints.test.mjs
• node test/plugin-manifest-regression.mjs
• npm test