Treat missing optional source-control CLIs as expected diagnostics states

[badcuban]

Priority: P1

Why

Settings -> Diagnostics currently shows failures when optional source-control tools are not installed. Missing tools like Azure CLI, GitLab CLI, or Jujutsu are expected probe outcomes, but they are counted as recent failures through VcsProcess.run and processRunner.runProcessCore.

Diagnostics evidence

From C:\Users\Will\.t3\userdata\logs\server.trace.ndjson* on May 26, 2026:

• Server diagnostics showed 8 failures in the recent trace window.
• 3 were VcsProcess.run failures for source-control.discovery.probe: az --version, glab --version, and jj --version.
• 3 paired lower-level failures were processRunner.runProcessCore spawn failures for the same probes.
• Code already catches these in SourceControlDiscovery.probe / SourceControlProviderDiscovery.probeCli and returns status: missing, so the user-facing source-control result is expected while diagnostics still looks broken.

Acceptance criteria

• Missing optional VCS/source-control CLIs are represented as normal probe results, not diagnostics failures.
• Unexpected spawn failures still surface with actionable command, cwd, and cause details.
• The Diagnostics latest-failures list is not polluted by expected missing optional integrations.
• Add regression coverage for missing optional CLI probes.

[badcuban]

Completed in the 0.0.2-0.0.4 QOL work. Optional source-control CLI probes now return expected missing states instead of polluting diagnostics as failures. Covered by SourceControlDiscovery/ProviderDiscovery regression tests.