Fix stale Project Explorer entries after case-only rename on case-insensitive filesystems

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Description

Fixes a bug where renaming a folder so that only its letter case changes (e.g. Company -> company) on a case-insensitive filesystem (default macOS APFS) leaves a stale, duplicate entry in the Project Explorer file tree.

Root cause: the filesystem watcher's debouncer (crates/watcher) can emit ambiguous ModifyKind::Name(RenameMode::Any) events for macOS renames, one per path involved. The code decided whether each such path was a create or a delete by calling Path::exists(). On a case-insensitive filesystem, Path::exists() resolves case-insensitively, so for a case-only rename both the old-case and new-case paths report as existing. Both then get treated as creates, and neither as a delete, leaving the stale old-case entry behind permanently (collapsing/re-expanding the tree does not clear it, matching the report).

An ordinary rename (different name entirely) isn't affected, because the old path genuinely stops existing under any case.

Fix: added path_exists_with_exact_case, which checks the parent directory's actual listing for an exact-case match instead of relying on Path::exists()'s case-insensitive resolution, and used it at that call site.

Linked Issue

Fixes #15698

• Where appropriate, screenshots or a short video of the implementation are included below (especially for user-visible or UI changes). (Not applicable — this is a backend file-watcher fix with no UI changes.)

Testing

Added unit tests in crates/watcher/src/lib_tests.rs:

• path_exists_with_exact_case_matches_the_exact_case_only / path_exists_with_exact_case_is_false_for_a_missing_path: directly test the new helper's exact-case matching contract.
• case_only_rename_is_treated_as_delete_of_old_case_and_create_of_new_case: an end-to-end test of deduplicate_and_merge_raw_notifier_events reproducing the reported scenario (old-case path routed to deleted, new-case path routed to added).

Ran cargo test -p watcher, cargo clippy -p watcher --all-targets --all-features -- -D warnings, and rustfmt --check on the changed files — all pass. Also verified cargo build/cargo clippy still succeed for the downstream repo_metadata crate (with the local_fs feature) that consumes this watcher code.

• I have manually tested my changes locally with ./script/run (not possible in this sandboxed, headless environment — validated via the automated tests above instead)

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-BUG-FIX: Fixed a bug where renaming a folder so only its letter case changes (e.g. Company -> company) on a case-insensitive filesystem left a stale duplicate entry in the Project Explorer file tree.

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