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Add circuit breaker pattern to vMCP health monitoring (#3528)

* Add circuit breaker pattern to vMCP health monitoring

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.

Related-to: #3036

* changes from review

* changes from review

* changes from review

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Co-authored-by: taskbot <taskbot@users.noreply.github.com>

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