feat: add token-to-cookie endpoint for embedded chat WebSocket auth (#23280)

## Problem

The VS Code extension embeds the Coder agent chat UI in an iframe,
passing the session token via `postMessage`. HTTP requests use the
`Coder-Session-Token` header, but browser WebSocket connections **cannot
carry custom headers** — they rely on cookies. This causes all WebSocket
requests (e.g. streaming chat messages) to fail with authorization
errors in the embedded iframe.

## Solution

Add `POST /api/v2/users/me/session/token-to-cookie` — a lightweight
endpoint that converts the current (already-validated) session token
into a `Set-Cookie` response. The frontend embed bootstrap flow calls
this immediately after `API.setSessionToken(token)`, before any
WebSocket connections are opened.

### Backend (`coderd/userauth.go`, `coderd/coderd.go`)
- New handler `postSessionTokenCookie` behind `apiKeyMiddleware`.
- Reads the validated token via `httpmw.APITokenFromRequest(r)`.
- Sets an `HttpOnly` cookie with the API key's expiry, applying
site-wide cookie config (Secure, SameSite, host prefix) via
`HTTPCookies.Apply`.
- Returns `204 No Content`.

### Frontend (`site/src/pages/AgentsPage/EmbedContext.tsx`)
- `bootstrapChatEmbedSessionFn` now calls the new endpoint after setting
the header token and before fetching user/permissions.
- The cookie is in place before any WebSocket connections are opened.

## Security

- **No privilege escalation**: The token is already valid — this just
moves it from a header credential to a cookie credential.
- **POST only**: Avoids CSRF-via-navigation.
- **Same origin**: The iframe loads from the Coder server, so the cookie
applies to the correct domain.
- **HttpOnly**: The cookie is not accessible to JavaScript.

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coderd/apidoc/docs.go

@@ -7826,6 +7826,29 @@ const docTemplate = `{
                 ]
             }
         },
+        "/users/me/session/token-to-cookie": {
+            "post": {
+                "description": "Converts the current session token into a Set-Cookie response.\nThis is used by embedded iframes (e.g. VS Code chat) that\nreceive a session token out-of-band via postMessage but need\ncookie-based auth for WebSocket connections.",
+                "tags": [
+                    "Authorization"
+                ],
+                "summary": "Set session token cookie",
+                "operationId": "set-session-token-cookie",
+                "responses": {
+                    "204": {
+                        "description": "No Content"
+                    }
+                },
+                "security": [
+                    {
+                        "CoderSessionToken": []
+                    }
+                ],
+                "x-apidocgen": {
+                    "skip": true
+                }
+            }
+        },
         "/users/oauth2/github/callback": {
             "get": {
                 "tags": [