fix(ai-sdk): spawn the MCP server without requiring a build

[brianstrauch]

What

The ai-sdk sample's MCP client factory spawned the MCP server as node lib/mcp-server.js — a cwd-relative path into compiled output that neither npm start nor npm run start.watch produces, since both run the Worker straight from src via ts-node.

On a fresh clone, npm run workflow mcp failed on every attempt (retried forever) with:

MCPClientError: Connection closed
    at DefaultMCPClient.onClose (@ai-sdk/mcp/src/tool/mcp-client.ts:1263:19)
    at StdioClientTransport.DefaultMCPClient.transport.onclose (@ai-sdk/mcp/src/tool/mcp-client.ts:438:41)
    at ChildProcess.<anonymous> (@modelcontextprotocol/sdk/src/client/stdio.ts:152:31)
    at ChildProcess.emit (node:events:509:28)
    at maybeClose (node:internal/child_process:1124:16)

The child process exited immediately with MODULE_NOT_FOUND, the stdio transport fired onclose, and the pending initialize request was rejected. The message is generic enough that it reads like a broken MCP session rather than "the server script isn't there".

Changes

• ai-sdk/src/worker.ts — resolve the server with require.resolve('./mcp-server'), so the path is absolute (cwd-independent) and follows whichever output is actually running: src/mcp-server.ts under ts-node, lib/mcp-server.js when compiled. Spawn it with process.execPath (the same Node binary running the Worker) plus -r ts-node/register for the TypeScript case, and set the child cwd to the project root so ts-node finds this project's tsconfig.json.
• ai-sdk/src/mcp-server.ts — move the startup banner from console.log to console.error. stdout is the JSON-RPC channel of a stdio MCP server, so that line was being fed to the client's read buffer as a bogus message (swallowed as a parse error, but latent breakage).

No build step is required anymore, which matches the README's setup instructions.

Testing

temporal server start-dev + npm start + npm run workflow mcp against a fresh, unbuilt lib/:

Running mcp
Started workflow workflow-PdAVOTgNvQ1fHa0Bq9CQK
Lickitung is a Normal-type Pokémon known for its long tongue and playful nature. ...
  - **Own Tempo**: Prevents Lickitung from becoming confused.
  - **Oblivious**: Prevents Lickitung from being infatuated or taunted.
  - **Cloud Nine**: Negates the effects of weather during battle.

• testServer-listTools and testServer-callTool both succeeded on the first attempt — no retries, no Connection closed.
• Confirmed the spawned child was node -r .../ts-node/register .../ai-sdk/src/mcp-server.ts, and the banner landed on the Worker's stderr.
• Verified the compiled path too: require.resolve('./mcp-server') from lib/worker.js resolves to lib/mcp-server.js and takes the plain-node branch.
• Verified the ts-node branch works when the Worker is started from an unrelated cwd.
• npm run build and prettier --check pass.

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