fix: handle JSON parsing in Claude Code CLI responses (#6125)

[roomote]

Summary

This PR fixes the issue where Claude Code CLI responses in code mode were displaying unparsed JSON with escaped newlines (\n\n\n) instead of properly formatted text.

Problem

When Claude Code CLI returns responses (especially during errors or when output is truncated), the raw JSON was being displayed to users with escaped newline characters, making the output difficult to read and understand.

Solution

• Added logic to detect and parse string chunks that contain JSON assistant messages
• Extract and properly format text content from parsed messages
• Handle malformed JSON gracefully by displaying it as-is (fallback behavior)
• Refactored assistant message processing into a reusable method

Testing

• Added comprehensive tests for the new parsing behavior
• Tests cover successful parsing, malformed JSON handling, and mixed content types
• All existing tests continue to pass

Related Issue

Fixes #6125

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

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Important

Fixes JSON parsing in Claude Code CLI, adds parsing method, and updates tests for proper formatting and error handling.

• Behavior:
  • Fixes JSON parsing in Claude Code CLI responses to display formatted text instead of raw JSON with escaped newlines.
  • Adds attemptParseAssistantMessage() in claude-code.ts to parse JSON assistant messages.
  • Handles malformed JSON by displaying it as-is.
• Refactoring:
  • Refactors assistant message processing into processAssistantMessage() in claude-code.ts.
• Testing:
  • Adds tests in claude-code.spec.ts for JSON parsing, malformed JSON handling, and mixed content types.
  • Ensures all existing tests pass.

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