fix: preserve Voyage asymmetric embedding semantics

[Tosko4]

Summary

• send input_type="document" when initializing Voyage embeddings and embedding stored content
• send input_type="query" when embedding recall queries
• preserve provider-default behavior for encode() and every non-Voyage LiteLLM provider
• add regression coverage for initialization, query/document routing, and non-Voyage compatibility

Why

Hindsight already exposes Voyage models through litellm-sdk, but the adapter currently omits Voyage's asymmetric retrieval input_type.

Voyage's retrieval contract embeds corpus content as document and search text as query. Passing those roles through Hindsight keeps stored vectors and recall queries in the intended retrieval spaces instead of relying on the provider's general-purpose default.

Validation

• uv run python -m pytest -n 0 tests/test_litellm_sdk_embeddings.py -q (29 passed, 3 skipped)
• uv run ruff check hindsight_api/engine/embeddings.py tests/test_litellm_sdk_embeddings.py
• uv run ruff format --check hindsight_api/engine/embeddings.py tests/test_litellm_sdk_embeddings.py
• ./scripts/hooks/lint.sh
• end-to-end retain, recall, worker, and reranking validation in a 2048-dimensional Voyage deployment

Risk and impact

The behavioral change is restricted to LiteLLM model names beginning with voyage/. Other providers continue to omit input_type, and direct encode() callers retain provider-default behavior.

Deployments that already indexed content with a Voyage model while input_type was omitted should rebuild those stored embeddings when adopting this change, so document and query vectors consistently use the asymmetric retrieval contract.

Scope

Two files only:

• hindsight-api-slim/hindsight_api/engine/embeddings.py
• hindsight-api-slim/tests/test_litellm_sdk_embeddings.py

No schema, configuration, dependency, generated-client, or public API changes.

Rollout

No database migration is included. New Voyage embeddings use the correct role automatically after upgrade. Existing Voyage-backed indexes should be re-embedded as noted above.

Reviewer focus

Please verify that:

• initialization and stored content use document
• recall queries use query
• encode() and non-Voyage providers stay unchanged

PR #3090 also touches these two files for retry and timeout behavior. It is not a duplicate, but whichever change lands second may need a focused rebase.