fix: assemble Anthropic streamed tool_use blocks before replay - #6683

Summary

• Anthropic streams tool arguments as input_json_delta events. MaxKB was replaying those raw deltas on the next turn, which Anthropic rejects with Input tag 'input_json_delta' does not match any of the expected tags (and empty text blocks with text content blocks must be non-empty).
• Assemble completed tool_use.input from streamed fragments, drop input_json_delta / empty text blocks, and treat stop_reason=tool_use as a finish chunk.
• Adds regression tests that do not require an Anthropic account.

Fixes #6480

Test plan

• python -m unittest apps.application.flow.tests.test_anthropic_tool_content -v
• SIMPLE agent + custom tool + Anthropic model, stream: true: tool call completes and the follow-up turn is not a 400

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