feat: synchronise provider changes with WatchAIProviders (#27091)

## Why

PR #26797 was accidentally merged into the stale `graphite-base/26797`
branch instead of `main` (Graphite picked the wrong base), so its
changes never landed on `main`. This PR re-lands that work as a clean
cherry-pick onto the current `main`.

## What

Adds a `WatchAIProviders` streaming RPC to the `ProviderConfigurator`
service so a running standalone AI Gateway refetches its provider set
when the provider configuration changes. The server subscribes to
`AIProvidersChangedChannel` (published by the provider CRUD endpoints)
and forwards each event as a payload-free signal, plus one signal on
subscribe; the gateway calls `GetAIProviders` on each signal to rebuild
its pool. The aibridged API is bumped to v1.2.

Env-seeded providers don't need a signal: seeding finishes before coderd
serves the gateway connection, so the gateway's initial fetch already
reflects the seeded set.

## For reviewers

The change is split into two commits to make review easy:

1. **`feat: synchronise provider changes with WatchAIProviders`** is a
faithful cherry-pick of #26797, identical to the originally reviewed PR.
It is committed without pre-commit hooks because it does not build
against current `main` on its own.
2. **`fix: resolve cherry-pick conflicts against main`** contains only
the deltas needed to re-land on current `main`, and passes the full
pre-commit suite:
- `coderd/aibridged/proto/aibridged.pb.go` regenerated via the proto
make target (the cherry-picked copy was generated against the older
proto).
- `enterprise/cli/aigatewaystart.go` import block unioned; `main` added
`os` and `strings` while the PR added `sync`.
- Three `aibridgedserver.NewServer` test call sites that landed on
`main` after the original branch diverged now pass the new `pubsub`
argument.

Refs https://linear.app/codercom/issue/AIGOV-465

*This PR was produced by opencode (agent) using the
`anthropic/claude-opus-4-8` model, under human direction and review.*

coderd/aibridged.go

@@ -65,7 +65,7 @@ func (api *API) CreateInMemoryAIBridgeServer(dialCtx context.Context) (client ai
 	}()
 
 	mux := drpcmux.New()
-	srv, err := aibridgedserver.NewServer(api.ctx, api.Database, api.Logger.Named("aibridgedserver"),
+	srv, err := aibridgedserver.NewServer(api.ctx, api.Database, api.Pubsub, api.Logger.Named("aibridgedserver"),
 		api.AccessURL.String(), api.DeploymentValues.AI.BridgeConfig, api.ExternalAuthConfigs, api.Experiments, api.AISeatTracker)
 	if err != nil {
 		return nil, err