feat(core): Late resolve `GenerateContentConfig`s and reduce mutation.

[joshualitt]

Cleans up a hack in GeminiChat to handle changes that need to be applied last minute to the config.

[gemini-code-assist[bot]]

Summary of Changes

Hello @joshualitt, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refactors the model configuration and selection logic within the core GeminiChat functionality. The primary goal is to remove an existing 'hack' that involved last-minute adjustments to the thinkingConfig for specific models. By introducing a ModelConfigKey and centralizing configuration resolution, the changes streamline how model-specific settings and availability policies are applied, leading to a cleaner and more maintainable codebase.

Highlights

• Refactored Model Configuration: The applyModelSelection function and GeminiChat now utilize a ModelConfigKey object to resolve model configurations, centralizing and streamlining config management.
• Removed thinkingConfig Hack: The specific conditional logic that applied last-minute adjustments to thinkingConfig based on gemini-3 models within GeminiChat has been eliminated, simplifying the codebase.
• Improved abortSignal Handling: The abortSignal is now passed explicitly as a parameter to makeApiCallAndProcessStream, enhancing clarity and consistency in signal propagation.
• Streamlined applyModelSelection: The applyModelSelection function has been simplified to directly resolve the final GenerateContentConfig based on the provided ModelConfigKey, removing complex conditional logic for config updates.

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[github-actions[bot]]

Size Change: -1.13 kB (-0.01%)

Total Size: 22 MB

Filename	Size	Change
./bundle/gemini.js	22 MB	-1.13 kB (-0.01%)

ℹ️ View Unchanged

Filename	Size
./bundle/sandbox-macos-permissive-closed.sb	1.03 kB
./bundle/sandbox-macos-permissive-open.sb	890 B
./bundle/sandbox-macos-permissive-proxied.sb	1.31 kB
./bundle/sandbox-macos-restrictive-closed.sb	3.29 kB
./bundle/sandbox-macos-restrictive-open.sb	3.36 kB
./bundle/sandbox-macos-restrictive-proxied.sb	3.56 kB

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