feat: add in-memory transport for chatd -> aibridge routing (#25576)

### TL;DR

Introduces an in-process `TransportFactory` for aibridge so that chatd (coder-agent LLM traffic) can route requests through the aibridged handler without crossing the HTTP route or requiring a license entitlement check.

### What changed?

- Added a new `coderd/aibridge` package with a `TransportFactory` interface and a `Source` type for tagging the call site on request contexts. `SourceAgents` is defined as the constant for coder-agent traffic.
- Implemented `NewTransportFactory` in `coderd/aibridged/transport.go`, which returns an `http.RoundTripper` that dispatches requests to the aibridged handler in-process. The response body is streamed through an `io.Pipe` so SSE/NDJSON/chunked responses propagate token-by-token. Handler panics are recovered and surfaced as 500 responses, and context cancellation closes the pipe with the appropriate error.
- `RegisterInMemoryAIBridgedHTTPHandler` now also constructs a `TransportFactory` from the registered handler and stores it on `API.AIBridgeTransportFactory` (an `atomic.Pointer`), making it available to chatd without going through the license-gated HTTP route.
- Added `API.AIBridgeTransportFactory` as a public `atomic.Pointer[aibridge.TransportFactory]` field on `coderd.API`.

### How to test?

- `coderd/aibridged/transport_test.go` covers: transport creation, nil-handler errors, source attachment to context, header/status passthrough, streaming (SSE-style chunked writes visible before handler completion), context cancellation closing the body with an error, concurrent requests, handler panics producing 500s, and handlers that return without writing.
- `coderd/aibridge_test.go` verifies that `AIBridgeTransportFactory` starts as nil on AGPL coderd, can be stored and loaded atomically, and that the stored factory correctly dispatches requests through the stub handler.

### Why make this change?

Chatd needs to send LLM requests through aibridge in-process rather than via the external HTTP route, which is license-gated. The `TransportFactory` abstraction provides a clean seam: the entitlement check remains on the HTTP route for external callers, while in-process coder-agent traffic bypasses it through the factory. The `Source` type allows downstream handlers and logs to attribute traffic without gating behavior on the caller identity.

coderd/coderd.go

@@ -45,6 +45,7 @@ import (
 	"github.com/coder/coder/v2/buildinfo"
 	"github.com/coder/coder/v2/coderd/agentapi"
 	"github.com/coder/coder/v2/coderd/agentapi/metadatabatcher"
+	"github.com/coder/coder/v2/coderd/aibridge"
 	"github.com/coder/coder/v2/coderd/aibridge/prices"
 	"github.com/coder/coder/v2/coderd/aiseats"
 	_ "github.com/coder/coder/v2/coderd/apidoc" // Used for swagger docs.
@@ -2198,9 +2199,15 @@ type API struct {
 	// UsageInserter is a pointer to an atomic pointer because it is passed to
 	// multiple components.
 	UsageInserter *atomic.Pointer[usage.Inserter]
+	// AIBridgeTransportFactory, when non-nil, lets chatd route LLM requests
+	// through an in-process aibridge transport instead of calling upstream
+	// providers directly. Registered by coderd at startup once aibridged is
+	// wired in-memory.
+	AIBridgeTransportFactory atomic.Pointer[aibridge.TransportFactory]
 	// aibridgedHandler is the in-memory aibridge HTTP handler. Set by
-	// RegisterInMemoryAIBridgedHTTPHandler; read by the enterprise
-	// /api/v2/aibridge route (license-gated).
+	// RegisterInMemoryAIBridgedHTTPHandler; read both by the enterprise
+	// /api/v2/aibridge route (license-gated) and by the in-memory transport
+	// (used by chatd, license-exempt).
 	aibridgedHandler http.Handler
 
 	UpdatesProvider tailnet.WorkspaceUpdatesProvider