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

aibridge/internal/integrationtest/keypool_failover_internal_test.go

@@ -78,7 +78,7 @@ func TestOpenAI_KeyFailover(t *testing.T) {
 				successBody = fix.NonStreaming()
 			}
 
-			pool, err := keypool.New([]string{"k0", "k1"}, quartz.NewMock(t))
+			pool, err := keypool.New(config.ProviderOpenAI, []string{"k0", "k1"}, quartz.NewMock(t), nil)
 			require.NoError(t, err)
 
 			var requestCount atomic.Int32
@@ -185,7 +185,7 @@ func TestAnthropic_KeyFailover(t *testing.T) {
 		t.Run(tc.name, func(t *testing.T) {
 			t.Parallel()
 
-			pool, err := keypool.New([]string{"k0", "k1"}, quartz.NewMock(t))
+			pool, err := keypool.New(config.ProviderAnthropic, []string{"k0", "k1"}, quartz.NewMock(t), nil)
 			require.NoError(t, err)
 
 			var requestCount atomic.Int32