fix: avoid mutating Gemini generation config

[lntutor]

Describe your changes

Copy the caller-owned generation_config before translating OpenAI-compatible option names to Gemini names. This keeps request preparation from deleting keys such as max_tokens from reusable caller state.

Adds a deterministic regression test and an Unreleased changelog entry.

Issue ticket number and link

Closes #2465

Validation

• uv run pytest -q tests/v2/test_gemini_utils_deterministic.py (19 passed)
• uv run ruff check instructor/v2/providers/gemini/utils.py tests/v2/test_gemini_utils_deterministic.py
• uv run ruff format --check instructor/v2/providers/gemini/utils.py tests/v2/test_gemini_utils_deterministic.py
• uv run ty check instructor/v2/providers/gemini/utils.py tests/v2/test_gemini_utils_deterministic.py

Checklist before requesting a review

• I have performed a self-review of my code
• If it is a core feature, I have added thorough tests.
• If it is a core feature, I have added documentation.

[jxnl]

Consolidated and shipped in #2495. Closing this focused patch as superseded; thank you for the contribution.