feat: add ai-gateway start command (#26605)

> AI Tools were used to produce this PR

This PR adds `coder ai-gateway start` command that runs the AI Gateway
as an independent process.

- Standalone process doesn't have access to DB. Uses DRPC services under
`/api/v2/ai-gateway/serve`for auth, recording and provider
initialization.
- It only handles LLM traffic, other endpoints (eg. `/sessions`) are
only available though `coderd`.
- The standalone gateway reuses applicable flags from AI Gateway
deployment options. Provider-seeding and coderd-only options are
excluded.
- Only added to fat build, the slim build stub rejects the command.

Some wiring used by this new command is added.

**`NewWebsocketDialer`** - implements the standalone gateway's
connection to coderd's `/api/v2/ai-gateway/serve` endpoint. It upgrades
to a WebSocket, multiplexes with yamux, and wires all DRPC services.

**`AIGatewayDataPlaneMiddleware`** - extracts the per-request middleware
chain (concurrency limiting, rate limiting, BYOK gating) into a shared
function used by both the embedded route and the standalone gateway.

**`RootCmd.ResolveClientConnection`** - resolve the deployment URL and
builds an HTTP transport without requiring a session token. Used in
`ai-gateway start`command as it authenticates using different credential
type.

---------

Co-authored-by: Danny Kopping <danny@coder.com>

coderd/aibridged/proto/version.go

@@ -17,6 +17,11 @@ const (
 	CurrentMinor = 1
 )
 
+// VersionQueryParam is the URL query parameter the standalone AI Gateway
+// uses to advertise its aibridged API version when dialing coderd's serve
+// endpoint, and that coderd reads to negotiate compatibility.
+const VersionQueryParam = "version"
+
 // CurrentVersion is the current aibridged API version.
 // Breaking changes to the aibridged API **MUST** increment CurrentMajor above.
 // Non-breaking changes to the aibridged API **MUST** increment CurrentMinor