Add size to DiskState to detect file changes

[ImBIOS]

Summary

This fix addresses the cross-platform root cause identified in issue #38109 where open buffers go stale or empty when external tools write files.

The Problem

The buffer's file_updated() method was only comparing mtime to determine if a buffer needed to be reloaded. This caused a race condition when external tools write files using std::fs::write(), which uses O_TRUNC and creates a brief window where the file is 0 bytes:

1. Scanner re-stats → sees 0 bytes, mtime T
2. file_updated() sees mtime changed → emits ReloadNeeded
3. Buffer reloads to empty, stamps saved_mtime = T
4. Tool finishes writing → file has content, but mtime is still T (or same-second granularity)
5. Scanner re-stats → mtime T matches saved_mtime → no reload triggered
6. Buffer permanently stuck empty

The Fix

Release Notes:

• Add the file size to DiskState::Present, so that even when mtime stays the same, size changes (0 → N bytes) will trigger a reload. This is the same fix that was identified in the issue by @lex00.

Changes

• crates/language/src/buffer.rs: Add size: u64 to DiskState::Present, add size() method
• crates/worktree/src/worktree.rs: Pass size when constructing File and DiskState::Present
• crates/project/src/buffer_store.rs: Pass size when constructing File
• crates/project/src/image_store.rs: Pass size when constructing File
• crates/copilot/src/copilot.rs: Update test mock

Test plan

• Open a file in Zed
• Write to that file from an external tool (e.g., echo "content" > file)
• Verify the buffer updates correctly without needing to reload

Fixes #38109

[lex00]

Hey @ImBIOS — I independently identified the same root cause and had a PR up for this back in February (#48691, opened Feb 7). I'm refiling it now with a rebased branch. My version includes integration tests covering both the truncate-then-write race and the mtime-differs recovery path, plus threading the size through image_store and copilot callers.

Happy to coordinate — let me know if you'd prefer to merge efforts or if you'd like me to reference your PR from mine.

[probably-neb]

Thanks for the fix @ImBIOS! Sorry it took so long for me to review.

[nojaf]

This didn't make the https://zed.dev/releases/stable/0.227.1 cut right?