feat: add key pool failover metrics to aibridge (#25901)

## Description

This PR adds Prometheus metrics for aibridge's API-key failover, giving visibility into key pool health and failover behavior per provider.

The following metrics are introduced:

- **`key_pool_state`** (gauge): number of keys currently in each state (`valid`, `temporary`, `permanent`) per provider, sampled at scrape time.
- **`key_pool_state_transitions_total`** (counter): key state transitions during failover, labeled by `reason` (`rate_limited`, `unauthorized`, `forbidden`).
- **`key_pool_exhaustions_total`** (counter): times a pool ran out of usable keys, labeled by `outcome` (`rate_limited`, `auth_failed`).
- **`key_pool_failover_attempts`** (histogram): keys attempted before success or exhaustion (per interception for bridged requests, per request for passthrough).

## Changes

- Moves `MarkKeyOnStatus` and key-pool error handling onto `*keypool.Pool`.
- Attaches metrics to each provider's key pool at install time, on construction and on provider reload.
- Adds a scrape-time state collector and a `KeyPools()` accessor on the bridge pool to feed it.
- Tracks per-request key attempts in the bridged and passthrough failover paths.
- Adds test coverage for the new metrics across the keypool unit tests, the bridged intercept failover tests, and the passthrough failover test.

Closes https://github.com/coder/internal/issues/1447
Closes https://linear.app/codercom/issue/AIGOV-198/aibridge-key-failover-observability

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

cli/server_aibridge_internal_test.go

@@ -744,7 +744,7 @@ func TestBuildAIProviderFromRowSetsAPIDumpDir(t *testing.T) {
 			provider, err := buildAIProviderFromRow(tt.row, nil, codersdk.AIBridgeConfig{
 				AllowBYOK:  serpent.Bool(true),
 				APIDumpDir: serpent.String(dumpDir),
-			})
+			}, nil)
 			require.NoError(t, err)
 			assert.Equal(t, dumpDir, provider.APIDumpDir())
 			assert.Equal(t, tt.expectedType, provider.Type())
@@ -762,7 +762,7 @@ func TestBuildAIProviderFromRowBedrockWithoutSettings(t *testing.T) {
 		BaseUrl: "https://bedrock-runtime.us-east-1.amazonaws.com/",
 	}, nil, codersdk.AIBridgeConfig{
 		AllowBYOK: serpent.Bool(true),
-	})
+	}, nil)
 	require.Error(t, err)
 	assert.Contains(t, err.Error(), "bedrock provider has no bedrock credentials configured")
 }