fix: keep Gemini functionResponse name/id paired

[wcqqq1214]

Fixes #9760.

Gemini tool-result messages only carry tool_call_id, so _prepare_conversation was using that id as functionResponse.name. Strict Gemini backends then reject the follow-up request (functionResponse.name "call_176130" does not match functionCall.name "gemini_search"), and they also require functionResponse.id when the matching functionCall has a distinct id.

A second failure showed up on the same follow-up path: an empty assistant content was serialized as an empty text part (contents[n].parts[0].data: required oneof field 'data' must have one initialized field). Tool-only first turns hit this often.

Modifications / 改动点

• Index assistant tool_calls before building Gemini contents so functionResponse.name is the original function name, even if a tool message appears before its call in the payload.

• Round-trip functionCall.id / functionResponse.id only when the id is distinct from the function name, so official Gemini (which often omits id, or uses the function name as a fallback) does not receive the field that previously caused [Bug] Gemini 模型工具调用报错 400: Unknown name "id" at 'function_response' #7357 / fix: remove unsupported id field from Gemini FunctionResponse #7386.

• Skip empty assistant text parts so a tool-only model turn does not produce an invalid Part.

• This is NOT a breaking change. / 这不是一个破坏性变更。

Verification Steps / 验证步骤

1. Ran ruff format --check and ruff check on astrbot/core/provider/sources/gemini_source.py.
2. Ran pytest tests/test_gemini_source.py (3 passed).
3. Exercised _prepare_conversation locally for:
   • the reported call_176130 / gemini_search pairing
   • a tool message appearing before its assistant tool_calls
   • official Gemini id == name, which still omits id
4. Live two-round tool call with gemini-3.7-flash on a backend that strictly validates function-call history:
   • Round 1: model called get_weather with a distinct id (call_1517).
   • Prepared contents paired functionCall / functionResponse as name=get_weather, id=call_1517, with no empty text part.
   • Round 2 text_chat with the tool result succeeded and used the tool output (30°C, sunny).
   • A no-tool prompt also succeeded.

Screenshots or Test Results / 运行截图或测试结果

tests/test_gemini_source.py ... 3 passed

Prepared Gemini contents for the reported shape:

function_call:     {id: call_176130, name: gemini_search, ...}
function_response: {id: call_176130, name: gemini_search, response: {name: gemini_search, content: ok}}

When tool_call_id equals the function name (get_weather), both parts omit id.

Live gemini-3.7-flash tool round-trip:

ROUND1  role=tool  name=get_weather  id=call_1517  args={city: Beijing}
PREPARED
  ModelContent  function_call:     {id: call_1517, name: get_weather, ...}
  UserContent   function_response: {id: call_1517, name: get_weather, response: {name: get_weather, content: "temperature=30C, condition=Sunny"}}
ROUND2  "The current weather in Beijing is sunny with a temperature of 30°C."

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Checklist / 检查清单

• 😊 If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc.
  / 如果 PR 中有新加入的功能，已经通过 Issue / 邮件等方式和作者讨论过。

• 👀 My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
  / 我的更改经过了良好的测试，并已在上方提供了“验证步骤”和“运行截图”。

• 🤓 I have ensured that no new dependencies are introduced, OR if new dependencies are introduced, they have been added to the appropriate locations in requirements.txt and pyproject.toml.
  / 我确保没有引入新依赖库，或者引入了新依赖库的同时将其添加到 requirements.txt 和 pyproject.toml 文件相应位置。

• 😮 My changes do not introduce malicious code.
  / 我的更改没有引入恶意代码。

Summary by Sourcery

Fix Gemini tool-result conversation serialization so strict backends accept paired function calls and responses.

Bug Fixes:

• Preserve matching Gemini function names and distinct IDs across function-call and function-response tool history.
• Avoid serializing empty assistant text parts that produce invalid Gemini requests.

Tests:

• Verify Gemini conversation preparation and tool round-trips with automated and live validation.

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

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