fix(devserver): resolve bridge port polling when controller reuses existing DevServer

[Copilot]

Summary

When the DevServer CLI MCP bridge launches a host process and that process exits
(e.g., a controller that detected an existing DevServer started by Rider/VS),
the bridge was polling its own randomly-chosen port instead of the existing
server's port — resulting in a 30-second timeout and false HostCrashed status.

Closes #22952

What changed

Core fix — AmbientRegistry fallback (DevServerMonitor.cs):

• After the host process exits during the readiness probe, re-query the
  AmbientRegistry for an active server for the same solution on a different port
• If found, adopt that server's port and retry the readiness probe on the correct port
• Clear _serverProcess so DeterminePostStartupAction correctly routes to
  HTTP health polling

Backward compatibility — 404 handling (DevServerMonitor.cs):

• WaitForServerReadyAsync now returns ReadinessProbeResult instead of bool
• Distinguishes between: Ready, ProcessExited, ServerRespondedNoMcp (HTTP
  alive but /mcp 404), TimedOut
• Hosts that respond to HTTP but lack /mcp report Degraded with an actionable
  diagnostic instead of HostCrashed

Stale state fix (DevServerMonitor.cs):

• HostRespondedNoMcp is reset in StopMonitoringAsync and before each
  readiness probe to prevent stale diagnostics across workspace transitions
  and crash recovery cycles

Decision model (MonitorDecisions.cs):

• ReadinessProbeResult enum for clean separation of probe outcomes
• ShouldAttemptAmbientFallback — extracted decision for testability
• IsReadinessAcceptable — extracted readiness gate for testability

Health diagnostics (HealthReport.cs, HealthReportFactory.cs, HealthService.cs):

• Added IssueCode.HostMcpEndpointNotAvailable (Warning severity)
• Surfaces remediation suggesting upgrading the DevServer package

Test plan

• Unit tests for ShouldAttemptAmbientFallback (5 cases: different port,
  no server, same port, wrong probe result, null solution)
• Unit tests for IsReadinessAcceptable (all 4 enum values)
• Unit tests for HostMcpEndpointNotAvailable health issue (present/absent)
• ReadinessProbeResult enum coverage
• 70/70 tests pass locally
• CI validation on hosted_ubuntu, macos, hosted_windows

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✅ carldebilly
❌ Copilot
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[jeromelaban]

Updated spec for this change?