feat: pass session token as query param on agent chat WebSockets (#23405)

## Problem

When the Coder chat UI is embedded in a VS Code webview, the session
token is set via the Coder-Session-Token header for HTTP requests.
However, browsers cannot attach custom headers to WebSocket connections,
and VS Code Electron webview environment does not support cookies set
via Set-Cookie from iframe origins. This causes all chat WebSocket
connections to fail with authorization errors.

## Solution

Pass the session token as a coder_session_token query parameter on all
chat-related WebSocket connections. The backend already accepts this
parameter (see APITokenFromRequest in coderd/httpmw/apikey.go).

The token is only included when API.getSessionToken() returns a value,
which only happens in the embed bootstrap flow. Normal browser sessions
use cookies and are unaffected.

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coderd/coderd.go

@@ -1517,7 +1517,6 @@ func New(options *Options) *API {
 				r.Post("/", api.postUser)
 				r.Get("/", api.users)
 				r.Post("/logout", api.postLogout)
-				r.Post("/me/session/token-to-cookie", api.postSessionTokenCookie)
 				r.Get("/oidc-claims", api.userOIDCClaims)
 				// These routes query information about site wide roles.
 				r.Route("/roles", func(r chi.Router) {