🚀 [firebase_ai] Public way to set thoughtSignature on request parts #18917

What feature would you like to see?

A public way to set thoughtSignature on the parts of a request.

Gemini 3 models return a thoughtSignature on response parts, and the API expects it back on those parts in later turns, otherwise multi-turn function calling with thinking fails. ChatSession handles this internally, but code that builds Content itself (for example a framework keeping its own message history) has no supported way to do it in firebase_ai 5.1.0:

• Part keeps the signature in a private _thoughtSignature. It can be read back through toJson(), but not set.
• The public TextPart, InlineDataPart, FileData and FunctionCall constructors pass thoughtSignature: null.
• Only the @visibleForTesting .forTest constructors accept it, and parsePart isn't exported.

We use the .forTest constructors in the Genkit plugin for Firebase AI Logic (genkit_firebase_ai) to round-trip signatures, which works but relies on test-only API that could change in any release.

Any of these would solve it:

1. A thoughtSignature named parameter on the public part constructors (and ideally a public thoughtSignature getter on Part).
2. Exporting parsePart, so a part can be rebuilt from the JSON the API returned.