fix: merge streamed agent message chunks to prevent fragmentation

[sshtomar]

When external agents (e.g., Claude Code) stream responses via WebSocket, each chunk was being rendered as a separate content block, resulting in fragmented display with gaps between chunks.

This change merges consecutive text blocks into a single block before rendering, providing a continuous, readable message display.

Fixes #6834

Solution

Added a mergeConsecutiveTextBlocks() helper function in frontend/src/components/chat/acp/blocks.tsx that:

1. Iterates through all content blocks from streamed chunks
2. Concatenates consecutive text blocks into a single unified text block
3. Preserves non-text blocks (images, audio, resources) in their original positions
4. Returns merged content for continuous display

Modified AgentMessagesBlock component to use this function before rendering, ensuring streamed text appears as a single, readable message.

Technical Details

• File modified: frontend/src/components/chat/acp/blocks.tsx
• Lines added: 47 (40 for the helper function + 7 for integration)
• Algorithm: Simple text accumulation with flush-on-non-text-block logic
• Backward compatible: Does not affect existing functionality or other message types

Additional Changes

The linter auto-fixed 12 other files for code style consistency (naming conventions, type syntax, numeric separators, copyright headers). These are purely formatting changes with no functional impact.

Checklist

• I have read the https://github.com/marimo-team/marimo/blob/main/CONTRIBUTING.md.
• For large changes, or changes that affect the public API: this change was discussed or approved through an issue, on https://marimo.io/discord?ref=pr, or the community https://github.com/marimo-team/marimo/discussions (Link:
  Agents panel splits streamed messages into many lines when connected to Claude Code via WebSocket #6834)
• I have added tests for the changes made. (Note: This is a UI fix for streaming behavior; manual testing performed with local marimo instance)
• I have run the code and verified that it works as expected.
  • Frontend builds successfully (make fe)
  • Linting and type checking pass (pnpm lint, pnpm typecheck)
  • Manual testing with local marimo server

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[github-actions]

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[sshtomar]

I have read the CLA Document and I hereby sign the CLA