fix: name run_code's required description argument in its model-facing prose

The transport schema requires both `code` and `description`, but the tool
description and both SDK instruction flavors described the call as passing a
program. `description` was reachable only through the parameter schema, so a
model following the prose emitted `{code}` alone and lost the whole written
program to an INVALID_ARGS rejection.

The length and format guidance stays in RUN_CODE_DESCRIPTION_PARAM_DESCRIPTION
alone, so the schema and the prompt cannot drift.

Fixes #2426

examples/acp-agent/tests/snapshots/code-mode-turn/system-prompt.expected.md

@@ -27,7 +27,7 @@ Use subagent in the background by default. Start independent delegations togethe
 
 ## Writing code for run_code
 
-Pass `run_code` the body of an async TypeScript function (erasable syntax only — no `enum` or namespaces; type annotations are advisory, the code runs type-stripped). Inside the program:
+`run_code` takes two required arguments: `code` — the body of an async TypeScript function (erasable syntax only — no `enum` or namespaces; type annotations are advisory, the code runs type-stripped) — and `description`, a short summary of what the program does. Inside the program:
 
 - Call tools as `await tools.name(args)` — quoted access for exotic names: `tools["my-tool"](args)`. Every call resolves to the tool's typed canonical JSON value. Tool arguments must be lossless JSON.
 - A FAILED tool call rejects with `ToolCallError`, whose `toolName` identifies the failed tool and whose `message` is human-readable — `try/catch` it to handle and continue.

examples/acp-agent/tests/snapshots/code-mode-turn/tool-schemas.expected.json

@@ -2,7 +2,7 @@
   "initial": [
     {
       "name": "run_code",
-      "description": "Execute a TypeScript program against the available tools. Write the BODY of an async function (erasable syntax only; top-level `await` and `return` work) and call tools as `await tools.name(args)` per the declarations in the system prompt. Only what you print or return comes back — curate it.",
+      "description": "Execute a TypeScript program against the available tools. Takes two required arguments: `code`, the BODY of an async function (erasable syntax only; top-level `await` and `return` work), and `description`, a short summary of what the program does. Call tools as `await tools.name(args)` per the declarations in the system prompt. Only what you print or return comes back — curate it.",
       "parameters": {
         "type": "object",
         "properties": {