[Bug] Fix dashboard backend build and socket tests on macOS

[JiaweiGithub]

Closes #4828

Purpose

Fix two macOS-only failures in dashboard/backend reported in #4828:

1. handlers/runtime_config_lock_unix.go passes stat.Mode directly to
   unix.Fchmod. The file builds for all non-Windows platforms
   (//go:build !windows), but unix.Stat_t.Mode is uint16 on darwin
   while unix.Fchmod expects uint32, so the whole handlers package
   fails to compile on macOS.
2. handlers/instance_test.go joins t.TempDir() with control.sock.
   t.TempDir() embeds the full test name, which exceeds the 104-byte
   sun_path limit for unix sockets on macOS, so
   TestInstanceUnixStatusAndModels and
   TestInstanceEngineUsesPersistentFrontendForInference always fail
   with bind: invalid argument.

Fixes: convert the mode with uint32(stat.Mode) (an identity conversion on
linux/freebsd where Mode is already uint32), and create the test socket
in a short os.MkdirTemp("", "sr") directory instead of t.TempDir().

Affected module: dashboard/backend only. No behavior change on Linux.
Windows is unaffected (file excluded by build tag).

Test Plan

cd dashboard/backend

# on main, macOS (darwin/arm64) — reproduces both failures
go build ./handlers/
go test ./handlers/ -run 'TestInstanceUnixStatusAndModels|TestInstanceEngineUsesPersistentFrontendForInference' -count=1

# with this PR
go build ./...
go test ./handlers/ -count=1

# cross-platform regression check
for p in 'linux amd64' 'linux arm64' 'darwin amd64' 'darwin arm64' 'freebsd amd64'; do
  GOOS=${p%% *} GOARCH=${p##* } go build ./handlers/ && \
  GOOS=${p%% *} GOARCH=${p##* } go vet ./handlers/
done

Test Result

Before (macOS, darwin/arm64):

handlers/runtime_config_lock_unix.go:62:33: cannot use stat.Mode & 0o777 | 0o060
(value of type uint16) as uint32 value in argument to unix.Fchmod

With only the compile fix applied, the two socket tests still fail:

listen unix /var/folders/.../TestInstanceEngineUsesPersistentFrontendForInference.../control.sock:
bind: invalid argument

After this PR:

• macOS: go build ./... passes; go test ./handlers/ -count=1 reports ok (all tests pass)
• go build and go vet pass on linux/amd64, linux/arm64, darwin/amd64, darwin/arm64, freebsd/amd64

Remaining risk: none expected; uint32(stat.Mode) is an identity conversion
on platforms where Mode is already uint32.

[Xunzhuo]

Thanks for fixing the macOS build and socket-path issues. One required Linux lint failure needs a scoped fix before this can land; details inline. Both Source Checks and Dashboard CI reproduce it. Darwin runtime tests were not run.

[Xunzhuo]

[P2] Keep the Darwin conversion lint-clean on Linux

The cast is needed on Darwin, but on Linux Stat_t.Mode is already uint32, so the required unconvert check rejects this line. Both Source Checks (make check) and Dashboard CI fail here at this commit, before the backend tests run. Please retain the portability fix and add a narrowly scoped //nolint:unconvert with the Darwin type rationale (or an equivalent platform-aware helper), then rerun both checks.

[JiaweiGithub]

Thanks for the review! Fixed in the latest commit.

[Xunzhuo]

Thanks for the focused lint fix. Both Darwin cross-builds and the Linux socket and lock regressions pass. Native macOS runtime tests were not run; no blockers found.

[wilsonwu]

LGTM