feat: add automatic key failover for AI Bridge Anthropic (#24836)

## Description

Adds automatic key failover for centralized Anthropic provider. When a key pool is configured, each upstream call walks the pool and tries keys in order until one succeeds or the pool is exhausted. Keys are marked **temporary** on 429 (with cooldown from `Retry-After`) and **permanent** on 401/403. Errors that aren't key-specific don't trigger failover. Each agentic-loop iteration gets its own fresh walker, so a tool-call continuation can fail over independently of the initial request.

BYOK is unchanged: BYOK requests run as a single attempt with no failover.

## Changes

- `config.Anthropic` carries a `KeyPool`. `Key` remains for BYOK X-Api-Key set per interception.
- Blocking interceptor: walks the pool, marks keys on key-specific failures, returns on first success or non-failover error.
- Streaming interceptor: per-iteration walker. Pre-stream failures fail over to the next key; mid-stream errors are relayed as SSE events.
- New `keypool` error types: `TransientExhaustionError` (carries soonest cooldown) and `ErrPermanentExhaustion`. Replace the prior `ErrAllKeysExhausted`.
- Error responses now consistently include the outer `"type": "error"` field.

## Related Issues

Related to: https://github.com/coder/internal/issues/1446
Related to: https://linear.app/codercom/issue/AIGOV-197/aibridge-automatic-key-failover-for-bridged-and-passthrough-routes

## Follow-up PRs

- Bedrock multi-key support.
- Refactor provider vs interceptor config separation.
- Record the actually-used key in the interception credential hint after failover.

> [!NOTE]
> Initially generated by Claude Opus 4.7, modified and reviewed by @ssncferreira

enterprise/cli/aibridged.go

@@ -10,10 +10,12 @@ import (
 
 	"github.com/coder/coder/v2/aibridge"
 	"github.com/coder/coder/v2/aibridge/config"
+	"github.com/coder/coder/v2/aibridge/keypool"
 	"github.com/coder/coder/v2/coderd/tracing"
 	"github.com/coder/coder/v2/codersdk"
 	"github.com/coder/coder/v2/enterprise/aibridged"
 	"github.com/coder/coder/v2/enterprise/coderd"
+	"github.com/coder/quartz"
 )
 
 func newAIBridgeDaemon(coderAPI *coderd.API, providers []aibridge.Provider) (*aibridged.Server, error) {
@@ -88,16 +90,24 @@ func buildProviders(cfg codersdk.AIBridgeConfig) ([]aibridge.Provider, error) {
 	}
 
 	// Add legacy Anthropic provider if configured. Bedrock credentials
-	// alone are sufficient — an Anthropic API key is not required when
+	// alone are sufficient, an Anthropic API key is not required when
 	// using AWS Bedrock.
 	if cfg.LegacyAnthropic.Key.String() != "" || getBedrockConfig(cfg.LegacyBedrock) != nil {
 		if _, conflict := usedNames[aibridge.ProviderAnthropic]; conflict {
 			return nil, xerrors.Errorf("legacy CODER_AIBRIDGE_ANTHROPIC_KEY conflicts with indexed provider named %q; remove one or the other", aibridge.ProviderAnthropic)
 		}
+		var pool *keypool.Pool
+		if key := cfg.LegacyAnthropic.Key.String(); key != "" {
+			var err error
+			pool, err = keypool.New([]string{key}, quartz.NewReal())
+			if err != nil {
+				return nil, xerrors.Errorf("create legacy anthropic key pool: %w", err)
+			}
+		}
 		providers = append(providers, aibridge.NewAnthropicProvider(aibridge.AnthropicConfig{
 			Name:             aibridge.ProviderAnthropic,
 			BaseURL:          cfg.LegacyAnthropic.BaseURL.String(),
-			Key:              cfg.LegacyAnthropic.Key.String(),
+			KeyPool:          pool,
 			CircuitBreaker:   cbConfig,
 			SendActorHeaders: cfg.SendActorHeaders.Value(),
 		}, getBedrockConfig(cfg.LegacyBedrock)))
@@ -110,14 +120,13 @@ func buildProviders(cfg codersdk.AIBridgeConfig) ([]aibridge.Provider, error) {
 		if name == "" {
 			name = p.Type
 		}
-		// Currently, only the first key is used, if any.
-		// TODO(ssncferreira): pass a keypool.Pool instead.
-		var key string
-		if len(p.Keys) > 0 {
-			key = p.Keys[0]
-		}
 		switch p.Type {
 		case aibridge.ProviderOpenAI:
+			// TODO(ssncferreira): pass a keypool.Pool instead.
+			var key string
+			if len(p.Keys) > 0 {
+				key = p.Keys[0]
+			}
 			providers = append(providers, aibridge.NewOpenAIProvider(aibridge.OpenAIConfig{
 				Name:             name,
 				BaseURL:          p.BaseURL,
@@ -127,10 +136,18 @@ func buildProviders(cfg codersdk.AIBridgeConfig) ([]aibridge.Provider, error) {
 				SendActorHeaders: cfg.SendActorHeaders.Value(),
 			}))
 		case aibridge.ProviderAnthropic:
+			var pool *keypool.Pool
+			if len(p.Keys) > 0 {
+				var err error
+				pool, err = keypool.New(p.Keys, quartz.NewReal())
+				if err != nil {
+					return nil, xerrors.Errorf("create anthropic key pool for provider %q: %w", name, err)
+				}
+			}
 			providers = append(providers, aibridge.NewAnthropicProvider(aibridge.AnthropicConfig{
 				Name:             name,
 				BaseURL:          p.BaseURL,
-				Key:              key,
+				KeyPool:          pool,
 				APIDumpDir:       p.DumpDir,
 				CircuitBreaker:   cbConfig,
 				SendActorHeaders: cfg.SendActorHeaders.Value(),