feat: add context limits for folder mentions to prevent overflow

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Related GitHub Issue

Closes: #11137

Description

This PR attempts to address Issue #11137. Feedback and guidance are welcome.

The Problem:
When a user uses @folder to mention a directory, the system reads ALL files in that folder without any limits. Even though individual files are truncated to 2000 lines, if a folder contains many files, the total content can explode and exceed the context window, causing errors on the first conversation.

The Solution:
This PR adds two limits to prevent context window overflow when processing folder mentions:

1. MAX_FOLDER_FILES_TO_READ (10): Limits the maximum number of files read from a folder
2. MAX_FOLDER_CONTENT_SIZE (100,000 characters / ~100KB): Limits the total content size from a folder mention

Key implementation details:

• Limits are checked before reading each file
• When limits are reached, users receive informative messages explaining the truncation
• Users are advised to use individual @file mentions instead of @folder for more control
• Folder listing is still shown in full (only content reading is limited)

Test Procedure

1. Unit tests were added to verify the new limits:

   • src/core/mentions/__tests__/folder-limits.spec.ts - 7 new tests

2. All tests pass:

   cd src && pnpm vitest run core/mentions/__tests__/folder-limits.spec.ts
   

3. Existing tests still pass:

   cd src && pnpm vitest run core/mentions/__tests__/index.spec.ts
   

4. Manual testing: Use @folder/ mention on a directory with many files and verify that:

   • Only up to 10 files are read
   • Total content is limited to ~100KB
   • A truncation notice appears informing the user

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue (see "Related GitHub Issue" above).
• Scope: My changes are focused on the linked issue (one major feature/fix per PR).
• Self-Review: I have performed a thorough self-review of my code.
• Testing: New and/or updated tests have been added to cover my changes (if applicable).
• Documentation Impact: I have considered if my changes require documentation updates (see "Documentation Updates" section below).
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Screenshots / Videos

N/A - Backend logic change, no UI impact.

Documentation Updates

• No documentation updates are required.

Additional Notes

The constants can be adjusted if needed:

• MAX_FOLDER_FILES_TO_READ = 10 - A conservative default that should work for most use cases
• MAX_FOLDER_CONTENT_SIZE = 100_000 - Approximately 100KB, which should be safe for most context windows

---

Important

Adds limits to folder mentions to prevent context overflow by restricting files read and content size, with tests to verify behavior.

• Behavior:
  • Adds MAX_FOLDER_FILES_TO_READ and MAX_FOLDER_CONTENT_SIZE constants in index.ts to limit files read and content size from folder mentions.
  • Updates getFileOrFolderContentWithMetadata() in index.ts to enforce these limits, skipping files and adding truncation notices when limits are reached.
• Tests:
  • Adds folder-limits.spec.ts with 7 tests to verify file and content size limits, truncation notices, and folder listing behavior.
• Misc:
  • Updates parseMentions() in index.ts to handle new truncation messages.

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Review complete. No issues found.

The implementation correctly adds context limits for folder mentions:

• File limit: 10 files max
• Size limit: ~100KB total content
• Clear truncation messages guide users to use individual @file mentions
• Full folder listing preserved even when content is truncated
• Tests comprehensive and passing

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