OCI Provider: Fix pydantic validation errors during tool call with streaming.

[gotsysdba]

Title

OCI streams tool calls progressively, so early chunks may be missing required fields. Add empty string defaults for missing fields before Pydantic validation happens. The fix is backward-compatible and only affects the specific case where OCI returns incomplete tool call data during streaming.

Relevant issues

Fixes #16897

Pre-Submission checklist

• I have Added testing in the tests/litellm/ directory, Adding at least 1 test is a hard requirement - see details
• I have added a screenshot of my new test passing locally
• My PR passes all unit tests on make test-unit
• My PR's scope is as isolated as possible, it only solves 1 specific problem

Type

🐛 Bug Fix
✅ Test

Changes

• Modified litellm/llms/oci/chat/transformation.py
  • Added logic to handle missing required fields in OCI streaming tool calls
  • The fix adds empty string defaults for missing arguments, id, and name fields before Pydantic validation
  • This prevents ValidationError when OCI streams incomplete tool call structures
• Test suite at tests/test_litellm/llms/oci/chat/test_oci_streaming_tool_calls.py
  • Test for missing arguments field
  • Test for missing id field
  • Test for missing name field
  • Test for all fields missing
  • Test for complete tool calls (regression test)
  • Test for multiple tool calls with mixed missing fields
  • Test for chunks without tool calls

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[gotsysdba]

Tests: