fix(gemini): strip Google model prefix

[kywch]

Summary

Normalizes google/gemini-* to the bare Gemini model ID only when configuring the Gemini ACP agent.

Root cause

Gemini CLI expects a bare model name. BenchFlow retained the models.dev google/ prefix, causing Gemini CLI to construct an invalid models/google/gemini-* path.

Impact

Provider-qualified Gemini model IDs work with the Gemini ACP agent; provider/model agents retain their existing prefix behavior.

Validation

• uv run python -m pytest tests/test_acp.py -k 'TestConnectAcpModelSelection'
• uv run ruff check src/benchflow/acp/runtime.py tests/test_acp.py
• git diff --check

Fixes #975.

[Galius5136]

I reproduced the model-prefix behavior from #975 locally and compared the merge-base (29c64e7), the isolated Gemini fix commit (771be99), and the current PR head (bc9a1c3).

On the merge-base, gemini + google/gemini-2.5-pro stays as google/gemini-2.5-pro. With the Gemini fix, it becomes gemini-2.5-pro, and the current PR head behaves identically to the isolated fix commit.

I also checked the surrounding non-regression cases: an already-bare Gemini model stays bare, OpenCode still maps gemini-2.5-pro to google/gemini-2.5-pro, and google/gemma-* is left unchanged for Gemini.

The ACP test suite passed on my setup (94 passed, 1 skipped), and the four files touched by the PR pass both ruff check and ruff format --check.

Everything behaved as expected in the offline path I tested. I didn't run a live Gemini CLI call because I don't have a GEMINI_API_KEY, so I didn't independently verify the bare model ID against the real Gemini CLI.

[kywch]

Thanks @Galius5136! I found other gemini issues as well, so I'd probably update on this thread.