fix(acp): convert RESOURCE_NOT_FOUND to ENOENT for fs delegation

[codefromthecrypt]

Summary

Fix ACP fs delegation error handling: convert RESOURCE_NOT_FOUND errors to
Node.js ENOENT format when reading files via client delegation.

Details

When clients advertise fs.readTextFile capability, gemini-cli delegates
file reads to the client. If the file doesn't exist, the client returns
an error with code -32002 (RESOURCE_NOT_FOUND). This change converts that
error to a Node.js-style ENOENT error so downstream code handles it correctly.

Changes:

• connection.ts: Add RESOURCE_NOT_FOUND error code constant
• fileSystemService.ts: Make capabilities public readonly, add try/catch
  in readTextFile() to convert RESOURCE_NOT_FOUND to ENOENT
• fileSystemService.test.ts: Add test for ENOENT conversion

How to Validate

1. Start gemini-cli with a client that advertises fs.readTextFile: true
2. Prompt to read a non-existent file
3. Verify the error has code: 'ENOENT' instead of raw JSON-RPC error

Pre-Merge Checklist

• Updated relevant documentation and README (if needed)
• Added/updated tests
• Noted breaking changes (if any)

Fixes #17588

[bdmorgan]

Hi @codefromthecrypt, thank you so much for your contribution to Gemini CLI! We really appreciate the time and effort you've put into this.

We're making some updates to our contribution process to improve how we track and review changes. Please take a moment to review our recent discussion post: Improving Our Contribution Process & Introducing New Guidelines.

Key Update: Starting January 26, 2026, the Gemini CLI project will require all pull requests to be associated with an existing issue. Any pull requests not linked to an issue by that date will be automatically closed.

Thank you for your understanding and for being a part of our community!

[codefromthecrypt]

@allenhutchison I added integration tests to reproduce the call sites based on the other PR you helped merge. if this is too much or anything lemme know

[gemini-cli]

Hi there! Thank you for your contribution to Gemini CLI.

To improve our contribution process and better track changes, we now require all pull requests to be associated with an existing issue, as announced in our recent discussion and as detailed in our CONTRIBUTING.md.

This pull request is being closed because it is not currently linked to an issue. You can easily reopen this PR once you have linked it to an issue.

How to link an issue:
Add a keyword followed by the issue number (e.g., Fixes #123) in the description of your pull request. For more details, see the GitHub Documentation.

Thank you for your understanding and for being a part of our community!

[codefromthecrypt]

@allenhutchison I backfilled the issue for this and linked it