fix(model): anthorpic stream thinking event

[rudy2steiner]

Summary
Fixes #1636 by parsing Anthropic streaming thinking_delta events into AgentScope ThinkingBlock content blocks. Non-streaming thinking blocks were already supported; this adds
parity for streamed responses.

Changes

• Added delta().thinking() handling in AnthropicResponseParser.parseStreamEvent(...).
• Converts Anthropic ThinkingDelta content into io.agentscope.core.message.ThinkingBlock.
• Added regression coverage for streaming thinking deltas using RawContentBlockDeltaEvent.builder().thinkingDelta(...).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!