BedrockEmbedder::embed doesn't validate response vector length #15

Summary: crates/embeddings/src/bedrock.rs::BedrockEmbedder::embed requests embeddings at self.dimensions but never checks that the returned vector's length actually matches self.dimensions before returning it. A mismatched (or empty) embedding — e.g. from a Bedrock model/version change, a partial response, or an API contract drift — propagates silently through the embedding pipeline and only fails later, at the Qdrant upsert step, with an error that doesn't point back to the actual cause.

Severity: S4 — latent/internal. This requires an external anomaly (Bedrock returning a vector of the wrong length) to manifest at all; it's not something a normal user or normal document triggers. The value of fixing it is fail-fast diagnostics, not preventing a currently-observed failure.

Environment:

• Deploy: any (dependent on Bedrock API behavior, not reproducible purely locally without mocking a malformed response)
• Commit: 6411ddb

Repro steps:

1. Mock/stub the Bedrock embeddings endpoint to return a vector shorter or longer than the configured dimensions (the existing test scaffolding for this module already mocks the HTTP layer — see embed_missing_embedding_field_maps_to_serialization_error and neighboring tests, crates/embeddings/src/bedrock.rs:153-183, as a pattern to extend).
2. Call BedrockEmbedder::embed(text).
3. Observe the mismatched-length vector is returned as Ok(...) with no validation error.

Expected: embed should reject a response whose vector length doesn't match self.dimensions, returning a clear EmbeddingError naming the mismatch, rather than propagating a bad vector downstream.

Observed: No length check exists between parsing the response and returning it (crates/embeddings/src/bedrock.rs:24-ish, embed); a length mismatch would only surface later at the Qdrant upsert boundary, with an error message that doesn't identify Bedrock as the source.

Evidence:

• Code: crates/embeddings/src/bedrock.rs:24-68-ish (embed), no comparison of the parsed vector's length against self.dimensions (set at construction, crates/embeddings/src/bedrock.rs:12-20).
• Existing tests (crates/embeddings/src/bedrock.rs:124- onward) cover missing-field, non-numeric, non-JSON, and service-error responses, but not a well-formed response with the wrong vector length.

Suspected area: crates/embeddings/src/bedrock.rs::BedrockEmbedder::embed — add a length check against self.dimensions immediately after parsing, returning a dedicated EmbeddingError variant.