fix: assemble Anthropic streamed tool_use blocks before replay

[lorenzozanee]

What this PR does / why we need it?

Anthropic streamed tool arguments were replayed as input_json_delta content blocks, which the Messages API rejects. This change assembles the fragments into completed tool_use.input blocks before replay and removes empty text blocks while preserving existing compatible tool-call handling.

Summary of your change

Fixes #6480 by normalizing Anthropic streamed tool content in the flow response handler.

Reviewers

@shaohuzhang1, @liuruibin, and @liqiang-fit2cloud

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.