test(ssh): use absolute stat paths and harden the test harness
Address code-review feedback:
- Stat the absolute "/test.git/config" path (memfs can resolve
relative vs absolute lookups differently); matches the rest of
the package.
- Close the listener in startSSHServer cleanup as well as the
server, avoiding a listener leak if Cleanup fires before the
concurrent Serve registers the listener.
- Quote the key path in GIT_SSH_COMMAND so paths with spaces are
not mis-split when git execs ssh.
- Rename the TestGitServiceFor loop variable tc -> tt for
package-wide consistency.
Assisted-by: Claude Opus 4.8 via Claude Code