fix: Expose ThinkingConfig class in firebase_ai.dart for use in GenerationConfig

[14T]

Expose ThinkingConfig class in firebase_ai.dart for use in GenerationConfig

Description

This PR adds the ThinkingConfig class to the export list in lib/firebase_ai.dart for the firebase_ai package. Currently, ThinkingConfig is defined in src/api.dart but not exported, causing a compilation error (The method 'ThinkingConfig' isn't defined) when users attempt to use it with GenerationConfig to configure the thinking budget for Vertex AI models (e.g., gemini-2.5-flash). This change ensures ThinkingConfig is accessible, enabling users to set the thinkingBudget parameter as documented in the Firebase AI Logic SDK (https://firebase.google.com/docs/ai-logic/thinking?api=vertex#set-thinking-budget).

Existing Behavior: The ThinkingConfig class is defined but not exported, making it inaccessible to users of the firebase_ai package despite being required for GenerationConfig.

Change: Added ThinkingConfig to the export 'src/api.dart' show ... statement in lib/firebase_ai.dart.

Motivation: Users encounter errors when trying to use ThinkingConfig to disable or configure the thinking budget, as intended by the SDK. This fix aligns the package with the documented functionality and improves developer experience.

Related Issues

• Resolves the issue where ThinkingConfig is not accessible, as reported by users attempting to use the thinking budget feature.

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See [Contributor Guide]). (Note: No behavior changes; export addition only. Tests will be added if required by reviewers.)
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///). (Note: Added doc comment for ThinkingConfig export if needed.)
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the [Flutter Style Guide].
• I signed the [CLA].
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.

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[ryanwilson]

Thanks so much for catching this @14T, and apologies for the issue!

[Lyokone]

Thanks @14T, it's now merged :)