feat: fetch providers over DRPC (#26650)

Closes [AIGOV-455](https://linear.app/codercom/issue/AIGOV-455/extend-drpc-with-buildproviders).

## Why

The AI Gateway (`aibridged`) is being split into a standalone process that must not touch the database. `coderd` stays the source of truth and seeds the `ai_providers` / `ai_provider_keys` tables from the environment. This PR adds a DRPC call so the gateway fetches provider config from `coderd` instead of reading the DB, for both the embedded and standalone daemons.

## What

- **Proto:** new `ProviderConfigurator` service with a unary `GetAIProviders` RPC, plus `AIProvider` / `AIProviderBedrock` messages. `CurrentMinor` bumped to 1 (additive).
- **Server (`coderd/aibridgedserver`):** `GetAIProviders` runs a read-only `InTx` under `LockIDAIProvidersEnvSeed` so it never returns a mid-seed snapshot, reads providers (incl. disabled) plus keys for enabled ones, and maps to proto under `dbauthz.AsAIBridged`. Unmappable rows are skipped and logged; plaintext keys and Bedrock secrets are never logged.
- **Client:** `DRPCProviderConfiguratorClient` wired into the client union, `dialer.go`, and `CreateInMemoryAIBridgeServer`.
- **cli:** `BuildProvidersFromProto` maps the response through the existing DB-neutral `buildProvider`. A shared `poolRPCReloader` does the fetch/build/replace for both daemons: the embedded daemon reloads on every `ai_providers` change and fails startup if it cannot subscribe; the standalone gateway drives the same reloader once at startup, retrying until success and staying interruptible.
- **Dead code removed:** `BuildProvidersFromConfig`, `ProvidersFromConfig`, `AIProviderFromConfig`, and the DB-read `BuildProviders` path.

enterprise/cli/aibridgeproxyd.go

@@ -11,7 +11,6 @@ import (
 
 	"golang.org/x/xerrors"
 
-	"cdr.dev/slog/v3"
 	"github.com/coder/coder/v2/aibridge/intercept/apidump"
 	"github.com/coder/coder/v2/coderd/aibridged"
 	"github.com/coder/coder/v2/coderd/database"
@@ -78,8 +77,10 @@ func newAIBridgeProxyDaemon(coderAPI *coderd.API) (io.Closer, error) {
 
 	unsubscribe, err := aibridged.SubscribeProviderReload(ctx, coderAPI.Pubsub, srv, logger.Named("provider-reload"))
 	if err != nil {
-		logger.Warn(ctx, "subscribe aibridgeproxyd to ai providers change channel", slog.Error(err))
-		unsubscribe = func() {}
+		// Without the subscription the proxy can never track provider changes,
+		// so fail startup rather than serve a permanently stale snapshot.
+		_ = srv.Close()
+		return nil, xerrors.Errorf("subscribe aibridgeproxyd to ai providers change channel: %w", err)
 	}
 
 	// Register the handler so coderd can serve the proxy endpoints.