## Summary

Fix agent message hyperlinks that point to Windows file paths but are
not parsed as openable project paths.

This covers links like:

```md
[Cargo.toml](</C:/Projects/Example Workspace/Cargo.toml:2>)
[filename.ext](C:\Projects\Example%20Workspace\path\to\filename.ext:42)
[AGENTS.md](</c/Projects/Example Workspace/AGENTS.md>)
```

## Problem

Agent responses can emit Markdown hyperlinks whose targets are Windows
paths rather than `file://` URLs. Some of those targets include a
leading slash before the drive (`/C:/...`), Git Bash/MSYS-style drive
prefixes (`/c/...`), percent-escaped spaces, or line suffixes. These
were not normalized before mention parsing, so clicking the hyperlink
could do nothing instead of opening the file.

## Solution

- Normalize hyperlink path targets before parsing them as `MentionUri`
paths.
- Decode percent escapes in bare path targets so `%20` becomes a literal
space before path/line parsing.
- Convert Windows-compatible hyperlink paths such as `/C:/...` and
`/c/...` into native Windows paths.
- Generate file resource links from `find_path_tool` through
`MentionUri::to_uri()` instead of hand-building `file://` strings.

## Result

Before: clicking agent path hyperlinks did not open the referenced file.

After:  


https://github.com/user-attachments/assets/6c7fad77-4a1e-4497-a4f9-4a4fdf86d527


## Validation

- `cargo test -p acp_thread test_parse_windows --features test-support`
- `cargo fmt --check`

## Follow-up changes

Additions on top of the original work above:

- Moved the hyperlink heuristics into a dedicated
`MentionUri::parse_hyperlink` entry point. `MentionUri::parse` stays
strict, so canonical mention URIs round-trip verbatim and other callers
(message editor, thread deserialization, resource links) are unaffected.
- Percent escapes that decode to path separators (`%2F`, `%5C`) are left
encoded, so decoding can never change which directories a path
traverses.
- Bare paths with escapes are ambiguous (a file may literally be named
`a%20b.rs`): `open_link` prefers the decoded interpretation and falls
back to `MentionUri::parse_hyperlink_literal` when the decoded path
doesn't resolve in the project but the literal one does.
- Links to files outside the project's worktrees now open, gated by an
async existence check through the project `Fs` (correct for remote
projects; broken links no longer create empty buffers or add worktrees).
`open_link` and the mention-crease open path are unified into one
`open_abs_path_at_point`, which now also places the cursor for
out-of-project selection/symbol links.
- `grep_tool` resource links also go through `MentionUri::to_uri()` now,
fixing malformed `file://C:\...` URIs and unencoded spaces.
- Added tests: percent-escape disambiguation, out-of-project link
opening, drive-letter normalization, UNC paths, and literal-path parsing
(`cargo test -p acp_thread mention`, `cargo test -p agent_ui open_link`,
`cargo test -p agent grep_tool`); manually verified the link spellings
above on Windows.

Release Notes:

- Fixed agent path hyperlinks on Windows when paths contain spaces or
shell-style drive prefixes.

---------

Co-authored-by: Martin Ye <martin@zed.dev>