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outdated headers, producing HTTP 429 "Update Required".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.

tests/__baseline__/providers-baseline.json

@@ -1,6 +1,6 @@
 {
   "alicode-intl": {
-    "baseUrl": "https://coding-intl.dashscope.aliyuncs.com/v1/chat/completions",
+    "baseUrl": "https://dashscope-intl.aliyuncs.com/compatible-mode/v1/chat/completions",
     "headers": {},
     "quirks": {
       "preserveCacheControl": true
@@ -231,7 +231,7 @@
       "Content-Type": "application/connect+proto",
       "User-Agent": "connect-es/1.6.1"
     },
-    "clientVersion": "3.1.0"
+    "clientVersion": "3.12.17"
   },
   "deepgram": {
     "baseUrl": "https://api.deepgram.com/v1/listen",