Fix GVFS.Mount exiting 0 on failure, hiding mount errors

[Tyrie Vella (tyrielv)]

Problem and Context

GVFS.Mount.exe could exit with code 0 even when a mount attempt failed, which hides the failure from anything that depends on its exit code. The root cause is in GVFS.Mount's entry point: Main was void, so the result of System.CommandLine's Invoke() call (the real outcome of the mount attempt) was discarded. The CLI action registered on the root command also only called verb.Execute() without returning its result.

Separately, a transient exception thrown while reading cache-server configuration from git config (CacheServerResolver.GetCacheServerFromConfig) could escape InProcessMountVerb.Execute() entirely uncaught. Because this happens after the mount log file is created but before anything is written to it, the result was an empty log with no explanation, on top of the process still exiting 0.

Together these mask real mount failures from any caller that checks gvfs mount's exit code, including the post-command hook that retries worktree mounts after certain git operations — a failed mount looks identical to a successful one, so a caller has no signal to retry or report an error.

Changes

• GVFS.Mount.Program.Main now returns int and propagates Invoke()'s result as the real process exit code, calling Environment.Exit only when it is non-zero (needed to force background threads to exit).
• InProcessMountVerb.BuildRootCommand()'s action now returns the real ReturnCode instead of being void: it catches MountAbortedException and returns its ReturnCode, since Invoke() never rethrows exceptions that escape the action — it always returns an int.
• InProcessMountVerb.Execute() now wraps the remainder of the mount attempt (including the cache-server config read) in a catch-all, so any exception that is not already a MountAbortedException is traced and reported instead of escaping silently with an empty log.
• MountVerb.Execute() no longer reports a child exit code of Success (0) when TryMount has already reported failure; it keeps the GenericError default in that case instead, since a child process reporting success while TryMount says otherwise is itself abnormal.
• GVFS.Hooks' TryMountWithRetry now prints the exit code, stdout, and stderr of each failed quiet retry to stderr, instead of discarding them silently.
• Added a regression test asserting that GVFS.Mount's root command returns GenericError (not 0 or a generic 1) for an invalid enlistment, and a WaitUntilMounted test case covering a mount process that exits with code 0.