Replace deprecated responseSchema with responseFormat for Gemini api

[Jazzcort]

This one is not urgent. Just replace the deprecated parameters.

I tested it with a prompt to ask the gatekeeper (Gemini) model to generate an invalid raw json string. Without responseFormat being set up, the model will always provide invalid json so it will hit the validation error when parsing the raw json to GatekeeperResult.

With the responseFormat being set up, the model will follow the schema we provide, so I think it's quite effective 😁

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
unittests	98.45% <100.00%> (-0.02%)	⬇️

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Files with missing lines	Coverage Δ
src/linux_mcp_server/gatekeeper/gemini_client.py	100.00% <100.00%> (ø)
tests/gatekeeper/test_gemini_client.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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