fix: restore URL content fetching for @url context menu mentions

[roomote-v0]

Related GitHub Issue

Closes: #11856

Description

This PR attempts to address Issue #11856 where the @url context menu option ("Paste URL to fetch contents") was not actually fetching any URL content. After the browser feature was removed, URL mentions in parseMentions() were only replaced with a quoted string reference without fetching the actual page content.

Changes:

• New fetchUrlContent utility (src/core/mentions/fetchUrlContent.ts): Uses axios for HTTP requests and cheerio for HTML text extraction. Strips scripts, styles, nav, footer, and other non-content elements, preferring <main>/<article> content. Handles non-HTML content types (plain text, JSON) as raw text. Includes timeout, redirect limits, and content length truncation.

• Updated parseMentions() (src/core/mentions/index.ts): Added URL mention handling in the mention processing loop. URL content is now fetched and included as a MentionContentBlock with type "url", consistent with how file/folder mentions work. Error handling provides informative messages when fetch fails.

• Updated text replacement: URL mentions now show "(see below for fetched content)" to indicate content will follow, matching the pattern used by other mention types.

No new dependencies were added -- axios and cheerio are already in package.json.

Feedback and guidance are welcome.

Test Procedure

• Added 7 tests in fetchUrlContent.spec.ts covering: HTML extraction, non-HTML content, JSON handling, content truncation, error propagation, body fallback, and missing content-type.
• Added 4 tests in index.spec.ts covering: text replacement, content block generation, error handling, and truncation indication.
• All 11 tests pass. Lint and type checks pass across the monorepo.

Run tests:

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

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.
• Documentation Impact: I have considered if my changes require documentation updates.
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Documentation Updates

• No documentation updates are required.

Additional Notes

The implementation uses a lightweight approach with axios + cheerio for URL content extraction instead of the previously removed browser/puppeteer-based approach. This avoids the complexity and resource overhead of a headless browser while still providing useful page content for most websites. JavaScript-rendered content will not be captured, but static HTML content (which covers most documentation, articles, and reference pages) will work correctly.

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