fix(cursor): HTTP/2 AgentService support + version bump to 3.12.17

Real Cursor IDE now uses AgentService at agent.api5.cursor.sh (HTTP/2-only)
while 9router still spoke the retired ChatService at api2.cursor.sh with
outdated headers, producing HTTP 429 "Update Required". Add an executeAgent
path that builds an agent.v1.RunRequest Connect RPC over a raw http2 stream
and fetches the account-specific usable model catalog via GetUsableModels.

Also implement MCP tool calling over AgentService: encode OpenAI tools as
AgentRunRequest.mcp_tools (McpToolDefinition with google.protobuf.Value
input_schema), decode McpArgs tool calls, and forward them to the client as
OpenAI tool_calls so the client runs the tool and resumes in the next turn.
Reply to request_context_args with a non-empty RequestContext, to server
heartbeats with client_heartbeat, and to KV blob get/set with empty results,
so action queries no longer stall the stream. Fold the client system prompt
into the user message (custom_system_prompt makes the server return an empty
turn). Bump clientVersion to 3.12.17 and add the x-cursor-client-commit
header so the gateway identifies as a current Cursor IDE release.

open-sse/providers/registry/cursor.js

@@ -23,7 +23,7 @@ export default {
       "Content-Type": "application/connect+proto",
       "User-Agent": "connect-es/1.6.1",
     },
-    clientVersion: "3.1.0",
+    clientVersion: "3.12.17",
   },
   models: [
     { id: "default", name: "Auto (Server Picks)" },
@@ -44,11 +44,11 @@ export default {
   oauth: {
     apiEndpoint: "https://api2.cursor.sh",
     chatEndpoint: "/aiserver.v1.ChatService/StreamUnifiedChatWithTools",
-    modelsEndpoint: "/aiserver.v1.AiService/GetDefaultModelNudgeData",
+    modelsEndpoint: "/agent.v1.AgentService/GetUsableModels",
     api3Endpoint: "https://api3.cursor.sh",
     agentEndpoint: "https://agent.api5.cursor.sh",
     agentNonPrivacyEndpoint: "https://agentn.api5.cursor.sh",
-    clientVersion: "3.1.0",
+    clientVersion: "3.12.17",
     clientType: "ide",
     dbKeys: {
       accessToken: "cursorAuth/accessToken",