Service: Fix automount regression - escape JSON arg passed to child mount processes

[tyrielv]

Symptom

Automount on the latest prerelease (v2.0.26154.1) silently failed for every
registered repo. The service log showed each GVFS.exe mount child being
started, then a 60-second WaitUntilMounted timeout and
"Unable to mount because the GVFS.Mount process is not responding". No
mount-side log file was created — the child died before the log listener
was attached.

Root cause

GVFSMountProcess.CallGVFSMount built the spawn command line by
concatenating InternalVerbParameters.ToJson() straight in:

"…\GVFS.exe" mount D:\os1 --internal_use_only {"ServiceName":null,…}

Windows CommandLineToArgvW strips the embedded " characters when
re-parsing the child's argv, so the receiving process sees
{ServiceName:null,…}. Newtonsoft.Json was permissive about unquoted
keys, so the bug was latent for years. After the
System.Text.Json migration,
strict parsing throws JsonException, the verb hits ReportErrorAndExit
before AddLogFileEventListener runs, and the child exits with no log.

Confirmed by reproducing the parse with CommandLineToArgvW:

argv[4] = '{ServiceName:null,StartedByService:true,MaintenanceJob:null,PackfileMaintenanceBatchSize:null}'

and feeding that to JsonDocument.Parse:

'S' is an invalid start of a property name. Expected a '"'.

Scope: only the SYSTEM-service automount path. gvfs mount (user-run)
and gvfs service --mount-all invoke MountVerb in-process — no
command-line round-trip, no bug.

Fix

• CommandLineEscaping.EscapeArgument — proper Windows command-line
  argument escaping per the CommandLineToArgvW rules (quote when
  needed, double trailing backslashes that precede a ", escape literal
  " as \").
• CurrentUser.RunAs → TryRunAs(string, string[], out int processId) —
  take argv as an array, escape centrally, return the child PID so
  callers can track its liveness.
• WindowsPlatform.StartBackgroundVFS4GProcess — route every arg
  through the new escaper (previously only space-containing args got
  naively wrapped, and embedded " were never escaped).
• GVFSEnlistment.WaitUntilMounted — new overload accepting
  Func<MountProcessSnapshot>. When supplied, the wait loop polls the
  snapshot between short (500 ms) connect attempts and fails fast with
  the exit code if the child has already terminated, instead of blocking
  for the full 60-second pipe timeout.
• GVFSMountProcess — pass the spawned PID into the new wait so
  logon automount detects an early GVFS.exe exit within seconds and
  logs "GVFS.Mount process (Id N) exited with code X before the named pipe was ready" instead of "not responding".

Tests

• CommandLineEscapingTests — canonical edge cases (spaces, tabs,
  embedded quotes, trailing backslashes) plus the exact
  InternalVerbParameters JSON shape that triggered the regression. A
  round-trip test feeds the escaped form back through
  CommandLineToArgvW and verifies the original string pops out
  unchanged.
• WaitUntilMountedProcessTrackingTests — verifies the wait
  short-circuits on an exit reported by the first snapshot poll, and
  also catches a late exit reported on a later poll, both well under
  the legacy 60-second timeout.

All 842 unit tests pass.

Validation checklist

• dotnet build GVFS\GVFS.UnitTests\GVFS.UnitTests.csproj -c Debug — 0 errors
• All unit tests green (842 passed, including 24 new)
• Manual smoke: install local Debug build, register two repos with
  gvfs mount, restart GVFS.Service, sign out / sign in, confirm
  both repos automount.
• Manual fault injection: temporarily make GVFS.exe exit non-zero
  in the mount verb and verify the service logs the new
  "GVFS.Mount process (Id N) exited with code X before the named pipe was ready" message within a few seconds rather than waiting
  60 s.