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

🔴 Required Information

Describe the Bug:
In the OpenAI-compatible (Chat Completions) path of ApigeeLlm, CompletionsHTTPClient._content_to_messages drops every non-tool part of a Content that also contains a function_response. Text parts are collected into content_parts, but the if function_responses: branch returns the tool messages (plus any tool-attached media) without ever using content_parts, so the text never reaches the model.

LiteLlm had the same issue and now sends the remaining parts as a follow-up message after the tool messages (lite_llm.py, _content_to_message_param).

Steps to Reproduce:

1. pip install google-adk==2.11.0
2. Run the snippet below.

Expected Behavior:
The tool message, followed by a user message carrying the text:

[{"role": "tool", "tool_call_id": "call_1", "content": "{\"ok\": 1}"},
 {"role": "user", "content": "Also, please summarize in French."}]

Observed Behavior:
The text is silently dropped:

[{"role": "tool", "tool_call_id": "call_1", "content": "{\"ok\": 1}"}]

Environment Details:

• ADK Library Version: 2.11.0 (also reproduced on main at 3d11f9f)
• Desktop OS: macOS
• Python Version: 3.12.13

Model Information:

• Are you using LiteLLM: No
• Which model is being used: ApigeeLlm in Chat Completions mode (any OpenAI-compatible backend)

🟡 Optional Information

Minimal Reproduction Code:

import json
from google.genai import types
from google.adk.models.apigee_llm import CompletionsHTTPClient

client = CompletionsHTTPClient(base_url="http://test")
content = types.Content(role="user", parts=[
    types.Part(function_response=types.FunctionResponse(
        id="call_1", name="f", response={"ok": 1})),
    types.Part(text="Also, please summarize in French."),
])
print(json.dumps(client._content_to_messages(content)))

How often has this issue occurred?:

• Always (100%)

Proposed fix: In the function_responses branch, append the remaining content_parts to the follow-up user message (alongside any tool-attached media), collapsing to a plain string when it is a single text part, as the non-tool branch already does. I have a PR ready with unit tests.