feat: add copilot gemini reasoning

[NateSmyth]

Issue

Now that GPT-5 models go to /responses and Anthropic models go to /v1/messages, Gemini is the last lobotomized copilot model left. This is a low(ish) footprint approach to let Gemini think.

Changes

Handles copilot's reasoning + reasoning_opaque replay. Most of this lives in a new CopilotChatLanguageModel, which handles transformation, storage, etc.
Re-uses some stuff from the /responses path where possible.
Also had to add a step to sanitize MCP tools, so extracted the relevant transform function for reuse.

Verify

Tested personally many times, both with Gemini sequentially as well as model/provider swaps.
The MCP issue originally came from chrome_devtools during testing, so I also made sure that works now.

Screenshots

---

Would fix#6864

Similar to #8900, but smaller diff. I figured it made sense as a different PR because routing Claude to /v1/messages changes things pretty significantly (although this approach also works with Claude).

---

Edit: also fixes#8417 (this is the "gemini prefers nullable keyword" issue)

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found the following potentially related PRs:

Related PRs Found:

1. PR feat(opencode): add copilot specific provider to properly handle copilot reasoning tokens #8900 - feat(opencode): add copilot specific provider to properly handle copilot reasoning tokens

   • feat(opencode): add copilot specific provider to properly handle copilot reasoning tokens #8900
   • Why related: The current PR (10963) explicitly mentions this PR in the description as similar work. PR feat(opencode): add copilot specific provider to properly handle copilot reasoning tokens #8900 handles Claude/Anthropic reasoning routing to /v1/messages, while PR feat: add copilot gemini reasoning #10963 handles the same for Gemini models.

2. PR feat: reasoning text for Gemini 3 Pro in GH Copilot #5346 - feat: reasoning text for Gemini 3 Pro in GH Copilot

   • feat: reasoning text for Gemini 3 Pro in GH Copilot #5346
   • Why related: Directly related to Gemini reasoning in GitHub Copilot context, which is the exact feature being added in PR feat: add copilot gemini reasoning #10963.

3. PR Feature/OpenAI compatible reasoning #5531 - Feature/OpenAI compatible reasoning

   • Feature/OpenAI compatible reasoning #5531
   • Why related: Part of the broader reasoning feature implementation for multiple AI providers.

Note: PR #8900 is the most directly related, as the current PR references it and builds on similar patterns for a different model provider (Gemini vs Claude).

[joseehilton147]

@rekram1-node can you check this?

[NateSmyth]

#11274 is a way simpler way to do this