feat(codersdk): add circuit breaker configuration support for aibridge (#21546)

## Summary

Add circuit breaker support for AI Bridge to protect against cascading
failures from upstream AI provider rate limits (HTTP 429, 503, and
Anthropic's 529 overloaded responses).

## Changes

- Add 5 new CLI options for circuit breaker configuration:
  - `--aibridge-circuit-breaker-enabled` (default: false)
  - `--aibridge-circuit-breaker-failure-threshold` (default: 5)
  - `--aibridge-circuit-breaker-interval` (default: 10s)
  - `--aibridge-circuit-breaker-timeout` (default: 30s)
  - `--aibridge-circuit-breaker-max-requests` (default: 3)
- Update aibridge dependency to include circuit breaker support
- Add tests for pool creation with circuit breaker providers

## Notes

- Circuit breaker is **disabled by default** for backward compatibility
- When enabled, applies to both OpenAI and Anthropic providers
- Uses sony/gobreaker internally via the aibridge library

## Testing

```
make test RUN=TestPoolWithCircuitBreakerProviders
```

cli/testdata/coder_server_--help.golden

@@ -121,6 +121,10 @@ AI BRIDGE OPTIONS:
           See
           https://docs.claude.com/en/docs/claude-code/settings#environment-variables.
 
+      --aibridge-circuit-breaker-enabled bool, $CODER_AIBRIDGE_CIRCUIT_BREAKER_ENABLED (default: false)
+          Enable the circuit breaker to protect against cascading failures from
+          upstream AI provider rate limits (429, 503, 529 overloaded).
+
       --aibridge-retention duration, $CODER_AIBRIDGE_RETENTION (default: 60d)
           Length of time to retain data such as interceptions and all related
           records (token, prompt, tool use).

cli/testdata/server-config.yaml.golden

@@ -777,6 +777,23 @@ aibridge:
   # these records to external SIEM or observability systems.
   # (default: false, type: bool)
   structuredLogging: false
+  # Enable the circuit breaker to protect against cascading failures from upstream
+  # AI provider rate limits (429, 503, 529 overloaded).
+  # (default: false, type: bool)
+  circuitBreakerEnabled: false
+  # Number of consecutive failures that triggers the circuit breaker to open.
+  # (default: 5, type: int)
+  circuitBreakerFailureThreshold: 5
+  # Cyclic period of the closed state for clearing internal failure counts.
+  # (default: 10s, type: duration)
+  circuitBreakerInterval: 10s
+  # How long the circuit breaker stays open before transitioning to half-open state.
+  # (default: 30s, type: duration)
+  circuitBreakerTimeout: 30s
+  # Maximum number of requests allowed in half-open state before deciding to close
+  # or re-open the circuit.
+  # (default: 3, type: int)
+  circuitBreakerMaxRequests: 3
 aibridgeproxy:
   # Enable the AI Bridge MITM Proxy for intercepting and decrypting AI provider
   # requests.