feat(gemini): 支持 Gemini Thinking 并持久化 thought signature

[wzl521]

Background

#2240 — The Gemini model adapter lacked first-class support for Gemini's built-in thinking capability. There were several gaps:

1. No includeThoughts / thinkingLevel options — GenerateOptions only had thinkingBudget, but Gemini's API requires includeThoughts (to independently control whether thoughts appear in responses) and thinkingLevel (e.g., "minimal",
   "low", "medium", "high") for the latest models.
2. thoughtSignature was not persisted — Gemini returns an opaque thoughtSignature on each part. This signature must be round-tripped back to Gemini on subsequent turns, but the codebase only captured it on ToolUseBlock (via
   ToolUseBlock.METADATA_THOUGHT_SIGNATURE), not on TextBlock or ThinkingBlock. After a JSON serialization round-trip, the byte-array signature was lost or corrupted.
3. Part boundaries were lost during accumulation — ThinkingAccumulator and TextAccumulator merged all chunks into a single block. When a Gemini response contained multiple parts each with its own thoughtSignature, the boundaries
   were destroyed, making it impossible to reconstruct distinct signed parts in the next request.

Changes

Core (agentscope-core)

• GenerateOptions: Added includeThoughts(Boolean) and thinkingLevel(String) options alongside the existing thinkingBudget. These are now properly merged in merge(GenerateOptions, GenerateOptions).
• ContentBlockMetadataKeys: New constant class with THOUGHT_SIGNATURE key, replacing the ad-hoc constant previously defined on ToolUseBlock. This is the single source of truth for the thought signature metadata key across all
  content block types.
• TextBlock: Added metadata field (provider-specific Map<String, Object>) with getter and builder support. This enables text blocks to carry thoughtSignature.
• ThinkingBlock: Already had metadata support; wired into the new signature persistence pipeline.
• ToolUseBlock: Replaced the local METADATA_THOUGHT_SIGNATURE constant with ContentBlockMetadataKeys.THOUGHT_SIGNATURE reference. No behavioral change.
• TextAccumulator / ThinkingAccumulator: Introduced buildAllTextBlocks() / buildAllThinkingBlocks() methods that preserve thought-signature Part boundaries. When a chunk carries a thoughtSignature, the current block is completed
  and a new one begins. The existing buildAggregated() method continues to return a single merged block for backward compatibility.
• ReasoningContext: Switched from buildAggregated() to buildAllXXXBlocks() for both text and thinking, ensuring the final assistant message preserves distinct signed parts.

Gemini Extension (agentscope-extensions-model-gemini)

• GeminiThoughtSignatureUtils (new): Utility class that:
  • Extracts thoughtSignature from a Gemini Part into content block metadata (as byte[] in memory).
  • Restores a persisted signature back onto a Gemini Part.Builder — supports both byte[] (in-memory) and Base64-encoded String (after JSON serialization).
• GeminiChatFormatter: Updated applyOptions to build ThinkingConfig from all three fields (thinkingBudget, thinkingLevel, includeThoughts). When only thinkingBudget is set (legacy callers), includeThoughts defaults to true
  preserving existing behavior. When includeThoughts is explicitly set, it always takes precedence.
• GeminiResponseParser: Refactored parsePartsToBlocks to:
  • Preserve original Part order (no longer reorders thinking → text → toolCall).
  • Attach thoughtSignature metadata to ALL block types (ThinkingBlock, TextBlock, ToolUseBlock) via GeminiThoughtSignatureUtils.extractMetadata.
  • Emit empty blocks when only metadata (signature) is present.
• GeminiMessageConverter: Updated to restore thoughtSignature from content block metadata onto Gemini Part objects for all assistant-role blocks (ThinkingBlock → Part.thought(true), TextBlock → text Part, ToolUseBlock →
  functionCall Part) via GeminiThoughtSignatureUtils.applyMetadata.

Documentation

• docs/v2/en/integration/model/gemini.md: Added "Thinking" section documenting thinkingLevel, includeThoughts, and the signature round-trip behavior.
• docs/v2/zh/integration/model/gemini.md: Chinese translation.
• docs/v2/en/integration/model/gemini-request-flow.md (new): Full end-to-end Mermaid diagram of a Gemini request, covering message conversion, thinking config, signature extraction/persistence, and the three completion paths (plain
  answer / tool call / persistent history replay).
• docs/v2/zh/integration/model/gemini-request-flow.md (new): Chinese translation.

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[codecov]

Codecov Report

❌ Patch coverage is 99.38650% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...model/gemini/formatter/GeminiMessageConverter.java	93.33%	0 Missing and 1 partial ⚠️

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[wzl521]

https://github.com/agentscope-ai/agentscope-java/issues/2240，please review，thanks

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