fix(anthropic): preserve thinking signatures

[logicwu0]

Summary

• preserve extended-thinking signatures in non-streaming and streaming responses
• accumulate signature metadata with streamed thinking text
• send signed thinking blocks back with the native Anthropic content type
• skip unsigned thinking blocks instead of exposing them as ordinary text

Testing

• mvn -pl agentscope-core -Dtest=ReasoningContextTest test
• mvn -pl agentscope-extensions/agentscope-extensions-model/agentscope-extensions-model-anthropic test
• mvn spotless:apply

Fixes #2400

[codecov]

Codecov Report

❌ Patch coverage is 97.22222% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...pe/core/agent/accumulator/ThinkingAccumulator.java	90.90%	0 Missing and 1 partial ⚠️

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