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outdated headers, producing HTTP 429 "Update Required".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/utils/cursorChecksum.js

@@ -128,7 +128,8 @@ export function buildCursorHeaders(accessToken, machineId = null, ghostMode = tr
     "x-amzn-trace-id": `Root=${crypto.randomUUID()}`,
     "x-client-key": clientKey,
     "x-cursor-checksum": checksum,
-    "x-cursor-client-version": "3.1.0",
+    "x-cursor-client-version": "3.12.17",
+    "x-cursor-client-commit": "0fb762053c34788bb7760d5673f8a6d4c8589d50",
     "x-cursor-client-type": "ide",
     "x-cursor-client-os": os,
     "x-cursor-client-arch": arch,