fix(openai): validate encoder input before embeddings call

[pandego]

Summary

• add input validation to OpenAIEncoder before calling the embeddings API
• fail fast with clear ValueError messages for:
  • empty input list
  • non-string list elements
  • blank or whitespace-only strings
• keep retry and API behavior unchanged for valid input

Why

Issue #530 reports OpenAI returning 400 '$.input' is invalid, which is difficult to diagnose from user code. This change catches common invalid input shapes earlier and surfaces clearer, actionable errors before the API call.

Validation

• uv run pytest tests/unit/encoders/test_openai.py
• result: 16 passed

Closes #530

[jamescalam]

good call - I'll have workflows run and merge when complete

[pandego]

Thanks! I checked the failing CI jobs.

The current failures are Pinecone router tests in tests/unit/test_router.py, and they appear unrelated to this patch. This PR only changes semantic_router/encoders/openai.py and tests/unit/encoders/test_openai.py.

[jamescalam]

yeah they're unrelated - I need to double check our CI pipeline envs @pandego