fix: validate embedding dimensions before pgvector writes

[ai-ag2026]

Summary

• validate embedding vectors before they can reach pgvector writes
• reject empty vectors and vectors whose length does not match the backend's declared dimension
• cover both retain and consolidation observation creation paths with regression tests

Root cause

Retain embedding generation already guaranteed that the number of returned vectors matched the number of input texts, but it did not validate per-vector dimensions. If an embedding backend returned [] or another wrong-sized vector, the invalid value could flow downstream until PostgreSQL/pgvector raised different vector dimensions 384 and 0.

The same shared embedding utility is also used by consolidation when creating observations, so the validation boundary protects both retain writes and consolidation-created observations.

Related reconnaissance

• Fixes batch_retain can pass zero-length embeddings to pgvector (vector(384) vs vector(0)) #1669.
• Related to Async operations need poison quarantine / degraded queue health #1671: without quarantine/degraded health, poisoned async operations can keep retrying even while /health is green.
• Related but separate from Sub-batch splitter announces 10K-token chunks but processes full payload as 1/1, triggering OOM on large single-item retains #1571, which tracks oversized single retain items not being split before retain processing.
• Existing tests already covered vector count mismatches from batch_retain IndexError: list index out of range in orchestrator.py _map_results_to_contents #1037; this PR extends the same boundary to validate each vector's shape.

Test plan

• RED on current origin/main: uv run pytest tests/test_consolidation_embedding_validation.py::test_create_observation_rejects_zero_length_embedding_before_insert -q -o addopts= failed because the zero-length vector reached the DB insert path.
• uv run pytest tests/test_consolidation_embedding_validation.py::test_create_observation_rejects_zero_length_embedding_before_insert tests/test_retain_orchestrator_mapping.py -q -o addopts= → 11 passed
• uv run ruff check tests/test_consolidation_embedding_validation.py hindsight_api/engine/retain/embedding_utils.py tests/test_retain_orchestrator_mapping.py → passed
• Added-line secret/private marker scan → 0 findings