Add circuit breaker pattern to vMCP health monitoring

[yrobla]

Implement circuit breaker state machine for backend health monitoring in Virtual MCP Server. The circuit breaker prevents cascading failures by tracking backend failures and transitioning through three states: Closed (normal operation), Open (failing), and HalfOpen (recovery testing).

Key changes:

• Add CircuitBreaker with thread-safe state machine
• Integrate circuit breaker with status tracker
• Integrate circuit breaker with health monitor
• Add comprehensive test coverage

The circuit breaker is disabled by default for backwards compatibility. Configuration wiring from YAML/CRD to health monitor is intentionally left for a follow-up PR to keep this change focused on the core implementation.

Large PR Justification

This is an atomic PR that exposes the circuit breaker capability. It includes comprehensive tests to validate it. Cannot be split as it's an isolated functionality.

Related-to: #3036

[github-actions]

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## Large PR Justification

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- Generated code that cannot be split
- Large refactoring that must be atomic
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[Copilot]

Pull request overview

This PR implements a circuit breaker pattern for backend health monitoring in the Virtual MCP Server to prevent cascading failures. The circuit breaker tracks backend failures and transitions through three states (Closed, Open, HalfOpen) to protect the system when backends are experiencing issues.

Changes:

• Adds CircuitBreaker implementation with thread-safe state machine for tracking failures and managing state transitions
• Integrates circuit breaker with StatusTracker and Monitor for health check gating
• Includes comprehensive test coverage for circuit breaker logic and integration scenarios

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
pkg/vmcp/health/circuit_breaker.go	New implementation of CircuitBreaker with state machine logic and thread-safe operations
pkg/vmcp/health/circuit_breaker_test.go	Comprehensive unit tests for circuit breaker state transitions, concurrency, and edge cases
pkg/vmcp/health/status.go	Integration of circuit breaker into status tracker with initialization, state querying, and health check gating
pkg/vmcp/health/status_test.go	Integration tests for circuit breaker with status tracker including disabled mode scenarios
pkg/vmcp/health/monitor.go	Circuit breaker configuration, initialization, and integration with health check loop
pkg/vmcp/health/monitor_test.go	End-to-end tests for circuit breaker behavior including disabled, enabled, recovery, and status reporting scenarios

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Large PR justification has been provided. Thank you!

[github-actions]

✅ Large PR justification has been provided. The size review has been dismissed and this PR can now proceed with normal review.

[codecov]

Codecov Report

❌ Patch coverage is 86.44860% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.68%. Comparing base (d45e969) to head (12d4d77).
⚠️ Report is 21 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/vmcp/health/monitor.go	70.27%	11 Missing ⚠️
pkg/vmcp/health/status.go	88.37%	7 Missing and 3 partials ⚠️
pkg/vmcp/health/circuit_breaker.go	91.20%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3528      +/-   ##
==========================================
+ Coverage   65.47%   65.68%   +0.21%     
==========================================
  Files         404      407       +3     
  Lines       39781    40195     +414     
==========================================
+ Hits        26046    26403     +357     
- Misses      11721    11754      +33     
- Partials     2014     2038      +24     

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[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

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