fix(LiteLlm): recognize assistant- prefix as valid OpenAI file ID

[nileshpatil6]

Azure OpenAI files uploaded with purpose=\"assistants\" receive IDs with an assistant- prefix (e.g. assistant-abc123). _looks_like_openai_file_id only checked for the file- prefix, causing Azure PDF attachments to be silently dropped from requests instead of being routed through the Responses API content block.

Fix: extend the startswith check to include \"assistant-\".

Also updated _redact_file_uri_for_log to return assistant-<redacted> for assistant- prefixed IDs, consistent with how file- IDs are handled.

Fixes #5664

[adk-bot]

Response from ADK Triaging Agent

Hello @nileshpatil6, thank you for creating this PR to fix the Azure OpenAI assistant prefix issue!

To help our reviewers process this contribution more efficiently and maintain code quality, could you please address the following items from our Contribution Guidelines:

1. Unit Tests / Testing Plan:
   • Please add or update unit tests to cover this new file ID prefix (assistant-) and verify that redaction behaves correctly under tests/unittests/.
   • Please include a testing plan section in your PR description detailing how you verified the changes (such as a summary of passed pytest results).
2. Logs or Screenshots:
   • As this is a bug fix, please provide logs or verification output showing how the file is successfully handled now compared to before.

These details will help ensure we prevent regressions and streamline the review process. Thank you again for your contribution!

[rohityan]

Hi @nileshpatil6 , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Can you please fix the failing unit tests before we can proceed with the review.