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
```

codersdk/deployment.go

@@ -3494,6 +3494,72 @@ Write out the current server config as YAML to stdout.`,
 			Group:       &deploymentGroupAIBridge,
 			YAML:        "structuredLogging",
 		},
+		{
+			Name:        "AI Bridge Circuit Breaker Enabled",
+			Description: "Enable the circuit breaker to protect against cascading failures from upstream AI provider rate limits (429, 503, 529 overloaded).",
+			Flag:        "aibridge-circuit-breaker-enabled",
+			Env:         "CODER_AIBRIDGE_CIRCUIT_BREAKER_ENABLED",
+			Value:       &c.AI.BridgeConfig.CircuitBreakerEnabled,
+			Default:     "false",
+			Group:       &deploymentGroupAIBridge,
+			YAML:        "circuitBreakerEnabled",
+		},
+		{
+			Name:        "AI Bridge Circuit Breaker Failure Threshold",
+			Description: "Number of consecutive failures that triggers the circuit breaker to open.",
+			Flag:        "aibridge-circuit-breaker-failure-threshold",
+			Env:         "CODER_AIBRIDGE_CIRCUIT_BREAKER_FAILURE_THRESHOLD",
+			Value: serpent.Validate(&c.AI.BridgeConfig.CircuitBreakerFailureThreshold, func(value *serpent.Int64) error {
+				if value.Value() <= 0 || value.Value() > 100 {
+					return xerrors.New("must be between 1 and 100")
+				}
+				return nil
+			}),
+			Default: "5",
+			Hidden:  true,
+			Group:   &deploymentGroupAIBridge,
+			YAML:    "circuitBreakerFailureThreshold",
+		},
+		{
+			Name:        "AI Bridge Circuit Breaker Interval",
+			Description: "Cyclic period of the closed state for clearing internal failure counts.",
+			Flag:        "aibridge-circuit-breaker-interval",
+			Env:         "CODER_AIBRIDGE_CIRCUIT_BREAKER_INTERVAL",
+			Value:       &c.AI.BridgeConfig.CircuitBreakerInterval,
+			Default:     "10s",
+			Hidden:      true,
+			Group:       &deploymentGroupAIBridge,
+			YAML:        "circuitBreakerInterval",
+			Annotations: serpent.Annotations{}.Mark(annotationFormatDuration, "true"),
+		},
+		{
+			Name:        "AI Bridge Circuit Breaker Timeout",
+			Description: "How long the circuit breaker stays open before transitioning to half-open state.",
+			Flag:        "aibridge-circuit-breaker-timeout",
+			Env:         "CODER_AIBRIDGE_CIRCUIT_BREAKER_TIMEOUT",
+			Value:       &c.AI.BridgeConfig.CircuitBreakerTimeout,
+			Default:     "30s",
+			Hidden:      true,
+			Group:       &deploymentGroupAIBridge,
+			YAML:        "circuitBreakerTimeout",
+			Annotations: serpent.Annotations{}.Mark(annotationFormatDuration, "true"),
+		},
+		{
+			Name:        "AI Bridge Circuit Breaker Max Requests",
+			Description: "Maximum number of requests allowed in half-open state before deciding to close or re-open the circuit.",
+			Flag:        "aibridge-circuit-breaker-max-requests",
+			Env:         "CODER_AIBRIDGE_CIRCUIT_BREAKER_MAX_REQUESTS",
+			Value: serpent.Validate(&c.AI.BridgeConfig.CircuitBreakerMaxRequests, func(value *serpent.Int64) error {
+				if value.Value() <= 0 || value.Value() > 100 {
+					return xerrors.New("must be between 1 and 100")
+				}
+				return nil
+			}),
+			Default: "3",
+			Hidden:  true,
+			Group:   &deploymentGroupAIBridge,
+			YAML:    "circuitBreakerMaxRequests",
+		},
 
 		// AI Bridge Proxy Options
 		{
@@ -3641,6 +3707,13 @@ type AIBridgeConfig struct {
 	MaxConcurrency      serpent.Int64           `json:"max_concurrency" typescript:",notnull"`
 	RateLimit           serpent.Int64           `json:"rate_limit" typescript:",notnull"`
 	StructuredLogging   serpent.Bool            `json:"structured_logging" typescript:",notnull"`
+	// Circuit breaker protects against cascading failures from upstream AI
+	// provider rate limits (429, 503, 529 overloaded).
+	CircuitBreakerEnabled          serpent.Bool     `json:"circuit_breaker_enabled" typescript:",notnull"`
+	CircuitBreakerFailureThreshold serpent.Int64    `json:"circuit_breaker_failure_threshold" typescript:",notnull"`
+	CircuitBreakerInterval         serpent.Duration `json:"circuit_breaker_interval" typescript:",notnull"`
+	CircuitBreakerTimeout          serpent.Duration `json:"circuit_breaker_timeout" typescript:",notnull"`
+	CircuitBreakerMaxRequests      serpent.Int64    `json:"circuit_breaker_max_requests" typescript:",notnull"`
 }
 
 type AIBridgeOpenAIConfig struct {