Distinguish container restarts from exits in monitor

[gkatz2]

Summary

• Add ErrContainerRestarted sentinel error to distinguish Docker restarts from true container exits
• Update the transport's handleContainerExit to reconnect the monitor and keep the proxy running when a restart is detected
• Fix double-close panic on shutdownCh and add mutex protection in reconnectMonitor

Fixes #3936

Context

The container monitor detects Docker restarts by comparing StartedAt times (#3843), but reports them with the same ErrContainerExited error as real exits. The transport layer can't tell them apart, so it always tears down the proxy and kills the already-running container. This causes cascading restart loops that take MCP servers offline — see #3936 for detailed impact.

Changes

• pkg/container/runtime/errors.go: New ErrContainerRestarted sentinel
• pkg/container/runtime/monitor.go: Send ErrContainerRestarted when start time changes
• pkg/container/docker/errors.go: Deprecated alias for backward compatibility
• pkg/transport/http.go: Loop-based handleContainerExit that reconnects the monitor on restart; reconnectMonitor with mutex protection; double-close guard in Stop()
• pkg/transport/http_test.go: Test case for ShouldRestart with ErrContainerRestarted
• pkg/transport/stdio.go: ShouldRestart excludes ErrContainerRestarted

Test plan

• Unit tests pass (go test ./pkg/container/runtime/... ./pkg/transport/...)
• Lint passes
• Manual test: started a server with patched binary, made MCP tool calls (initialize + tools/call), ran docker restart, confirmed proxy stayed up and MCP tool calls succeeded post-restart
• Manual test: docker stop still correctly tears down the transport

[codecov]

Codecov Report

❌ Patch coverage is 9.43396% with 48 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.64%. Comparing base (057d93d) to head (6a2b59a).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/transport/http.go	4.00%	48 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3937      +/-   ##
==========================================
+ Coverage   67.62%   67.64%   +0.01%     
==========================================
  Files         438      438              
  Lines       44108    44141      +33     
==========================================
+ Hits        29829    29857      +28     
- Misses      11945    11950       +5     
  Partials     2334     2334              

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