fix: tool-retrieval MCP server crashes on startup (plain-object schema bug)

[GusCayresMindsight]

Problem

`opencode-workspace mcp-serve` exited immediately on every invocation with:

```
tool-retrieval-server: fatal error: Schema is missing a method literal
MCP error -32000: Connection closed local mcp startup failed
```

`Server.setRequestHandler()` in `@modelcontextprotocol/sdk` requires a Zod schema as its first argument. The code was passing a plain JavaScript object instead:

```js
// Before — crashes the server on startup
server.setRequestHandler({ method: 'tools/list' }, handler);
server.setRequestHandler({ method: 'tools/call' }, handler);
```

The SDK calls `getObjectShape(schema)` on the first argument. A plain object has no `.shape` or `._zod` property, so `shape?.method` is `undefined` and the SDK throws synchronously — killing the process before it can serve a single request.

Fix

Use the correct Zod schemas exported by the SDK:

```js
const { ListToolsRequestSchema, CallToolRequestSchema } =
await import('@modelcontextprotocol/sdk/types.js');

server.setRequestHandler(ListToolsRequestSchema, handler);
server.setRequestHandler(CallToolRequestSchema, handler);
```

Refactor (enables the new tests)

• Extract server setup into `createMcpServer({ Server, ListToolsRequestSchema, CallToolRequestSchema })` — accepts its SDK dependencies explicitly so it can be unit-tested with a mock `Server` without binding to stdio.
• Remove the module-level `startServer()` auto-call; export `startServer()` and call it from `bin/cli.js`. This makes `require()`ing the module in tests safe.

Tests added

All previously existing 50 scenarios still pass. Three new scenarios cover the layer that was completely untested:

Scenario	What it catches
Zod schema validation	`setRequestHandler` is called with proper Zod schemas; each schema `.safeParse`s a valid request — would have failed with the old plain-object code
list-tools wire protocol	Full `Client↔Server` round-trip via `InMemoryTransport`; asserts `search_tools` appears in the manifest with `query` in `required`
call-tool wire protocol	Same in-process transport; asserts a valid `{ content, isError }` envelope is returned (uses empty corpus path — no embedder needed)

53/53 scenarios pass, 234/234 steps pass.