fix(model): preserve Anthropic streaming content blocks - #2495

AgentScope Version

2.0.7.post1

Description

Fixes #2494.

Anthropic streaming responses currently share one ID for all thinking blocks and one ID for all text blocks. This merges separate blocks, overwrites earlier thinking signatures, and changes their order relative to tool calls.

This PR assigns separate block IDs by event.index, preserving each block's content, signature, and position while continuing to accumulate deltas within the same block.

Regression tests cover:

• Multiple thinking blocks with separate signatures.
• Thinking blocks containing only signatures.
• Text blocks before and after a tool call.
• Thinking blocks separated by redacted thinking.
• Multiple deltas within one block.

The tests compare streaming and non-streaming results and verify the assistant message formatted for the next request.

Validation

• Related model, response aggregation, and Anthropic formatter tests: 80 passed, 10 subtests passed.
• pre-commit run --all-files: passed.
• The full test suite did not pass locally. Remaining failures include Redis/Lua, DingTalk, and MCP tests; Redis-backed tests reported missing Lua EVAL support.

Checklist

• An issue has been created for this PR.
• I have read CONTRIBUTING.md.
• Docstrings follow the project style.
• Documentation impact reviewed; no public API or configuration changes.
• Code is ready for review.