fix(models): keep text sent alongside a function response in ApigeeLlm

[AkshayShah03]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: ApigeeLlm (Chat Completions): text sent alongside a function_response is silently dropped #7475

Problem:
In the Chat Completions path of ApigeeLlm, CompletionsHTTPClient._content_to_messages returned early when a Content held a function_response, emitting only the tool messages (plus any tool-attached media). Text or media parts sent in the same Content were collected into content_parts but never used, so they were silently dropped from the request.

Solution:
A tool-role message carries only the tool result, so the remaining parts now follow the tool messages in a single user message, together with any tool-attached media (media first, then the other parts). A lone text part collapses to a plain string, matching the non-tool branch. This mirrors how LiteLlm already handles the same case (_content_to_message_param sends leftover parts as a follow-up message).

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Added to tests/unittests/models/test_apigee_llm.py:

• test_content_conversion_keeps_text_alongside_function_response: function response + text → tool message, then {"role": "user", "content": "<text>"}.
• test_content_conversion_keeps_text_alongside_function_response_media: function response with attached image + text → tool message, then one user message with the image and the text.

Both fail on main and pass with this change. The existing media-only and no-media tests are unchanged and still pass.

$ pytest tests/unittests/models/
1523 passed, 33 warnings in 11.95s

pyink and isort report no changes.

Manual End-to-End (E2E) Tests:

Not run against a live Apigee / OpenAI-compatible backend. The reproduction snippet from #7475 prints only the tool message on main and the tool message followed by the user text message with this change.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

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[AkshayShah03]

@googlebot I signed it!

[adk-bot]

Thank you @AkshayShah03 for your contribution! 🎉

Your changes have been successfully imported and merged via Copybara in commit f5db310.

Closing this PR as the changes are now in the main branch.