Preserve Gemini tool arguments and text thought signatures

[mattt]

Gemini conversation history currently wraps tool arguments in GeneratedContent’s persistence format and drops thought signatures on text parts. This PR replays arguments from their semantic JSON and preserve signed text with its original part boundaries and position among tool calls.

Fixes #235
Fixes #221

[Copilot]

🟢 Approval recommended

The implementation addresses both reported regressions with focused persistence changes and comprehensive request-level tests.

Pull request overview

Fixes Gemini transcript replay by preserving semantic tool arguments and signed text-part metadata across turns and serialization.

Changes:

• Replays tool arguments from semantic JSON.
• Persists and restores signed text-part boundaries.
• Adds streaming and transcript round-trip coverage.

File summaries

File	Description
Sources/AnyLanguageModel/Models/GeminiLanguageModel.swift	Preserves Gemini arguments and signed text parts.
Sources/AnyLanguageModel/LanguageModelSession.swift	Carries provider metadata into transcripts.
Sources/AnyLanguageModel/Transcript.swift	Adds persisted turn-level provider metadata.
Tests/AnyLanguageModelTests/GeminiThoughtSignatureTests.swift	Covers arguments, signatures, ordering, and streaming.
Tests/AnyLanguageModelTests/TranscriptTests.swift	Tests provider-metadata Codable behavior.
Tests/AnyLanguageModelTests/Shared/StubURLProtocol.swift	Adds SSE response support.

Review details

• Files reviewed: 6/6 changed files
• Comments generated: 0
• Review effort level: Balanced

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