Fix Windows unix socket URL round-trip

[stantheman0128]

Summary

I reproduced this on Windows 11 with go test ./pkg/api ./pkg/server/discovery. Drive-letter emit was already unix:///C:%5C.... What still failed:

• ListenURL for /tmp/test.sock came out as unix:////tmp/test.sock (extra slash).
• ParseUnixSocketPath("unix:///var/run/thv.sock") ran the path through filepath.Clean, so Windows turned it into \var\run\thv.sock and rejected it as not absolute.
• Health tests built "unix://"+socketPath, which url.Parse rejects on a drive letter (invalid port).

I put emit on one helper (discovery.UnixSocketURL) and taught the parser the two absolute forms instead of three one-off patches.

• POSIX paths keep three slashes. Drive-letter paths still get the synthetic leading slash that net/url needs.
• POSIX unix:/// stays slash-form on Windows. The old four-slash alias still parses.
• Health tests call the helper.

Fixes #6291

Type of change

• Bug fix
• New feature
• Refactoring (no behavior change)
• Dependency update
• Documentation
• Other (describe):

Test plan

• Unit tests (task test)
• E2E tests (task test-e2e)
• Linting (task lint-fix)
• Manual testing (describe below)

Win11, go1.26.5, worktree at origin/main e25de16. I did not run full task test (whole-repo -race). Focused:

go test -timeout 90s -count=1 ./pkg/server/discovery/
ok  	github.com/stacklok/toolhive/pkg/server/discovery	1.511s

go test -timeout 90s -count=1 ./pkg/api/ -run "TestListenURL|TestSocketURL"
ok  	github.com/stacklok/toolhive/pkg/api	4.847s

go test -timeout 60s -count=1 ./pkg/api/ -run "TestSetupUnixSocket|TestCleanupUnixSocket|TestCreateListener|TestIsNamedPipe"
ok  	github.com/stacklok/toolhive/pkg/api	1.817s

TestListenURL/Unix_socket_returns_unix_URL, TestUnixSocketURL, POSIX/drive-letter round-trip, and TestParseUnixSocketPath_FourSlashAlias are in that set.

API Compatibility

• This PR does not break the v1beta1 API, OR the api-break-allowed label is applied and the migration guidance is described above.

Changes

File	Change
pkg/server/discovery/unix_url.go	Shared UnixSocketURL plus POSIX vs drive-letter parse
pkg/server/discovery/health.go	ParseUnixSocketPath delegates to that parse
pkg/api/socket_windows.go / socket_unix.go	socketURL calls UnixSocketURL (npipe branch unchanged)
*_test.go	Round-trip, four-slash alias, health tests stop concatenating

Does this introduce a user-facing change?

Yes, for AF_UNIX discovery URLs. A POSIX socket path on Windows is now unix:///tmp/... (three slashes), not unix:////tmp/.... Drive-letter URLs are unchanged (unix:///C:%5C...). Named pipes are unchanged.

Special notes for reviewers

I left pkg/container/docker/sdk/client_unix.go (unix://+docker socket into moby WithHost) alone. That is a different host string, unix-only build.

I used Cursor to write the patch after reproducing the failures on this box.

Claimed and assigned on #6291.

[codecov]

Codecov Report

❌ Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 77.72%. Comparing base (e25de16) to head (213eefa).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/server/discovery/unix_url.go	96.42%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6416      +/-   ##
==========================================
+ Coverage   77.70%   77.72%   +0.01%     
==========================================
  Files         756      760       +4     
  Lines       72788    72907     +119     
==========================================
+ Hits        56563    56669     +106     
- Misses      16220    16233      +13     
  Partials        5        5              

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[samuv]

Thanks for the focused fix and the coverage for both POSIX and drive-letter round trips. I found one path-validation edge case where a drive-relative Windows path is accepted as absolute, so I left an inline comment. The current PR checks are green.

Checklist:

• Tests: New regression tests cover the intended paths, and CI is green.
• Docs: No additional docs needed for this internal compatibility fix.
• Registry impact: None.
• Security: One path-validation issue noted inline.
• Backwards compatibility: The legacy four-slash alias remains supported.