test: end-to-end test of a generated MCP server (gRPC backend + MCP client) #87

Part of #44. This is the behavioural safety net for the migration: it pins down what MCP clients see today, so the official-SDK version (#89) can be checked against it.

Do

Add an end-to-end test (e.g. examples/gen/example/v1/e2e_test.go, or test/e2e/) that:

1. Starts an in-memory gRPC server (google.golang.org/grpc/test/bufconn) with a fake VibeService that records the requests it receives and returns canned responses or errors.
2. Builds the generated MCP server (NewVibeServiceMCPServer + RegisterDefaultTools) on a client to that backend.
3. Talks MCP to it in-process, using mark3labs' in-process client if v0.21 has one, otherwise MCPServer.HandleMessage with JSON-RPC messages.
4. Asserts:
   • tools/list: the tool names, descriptions and input schemas (snapshot them).
   • tools/call for every RPC: the arguments sent reach the fake backend as the right proto request.
   • Results: the tool result content matches the response.
   • Errors: a gRPC error (e.g. codes.NotFound) produces a tool result with isError: true.

Keep the MCP-client part behind one small helper (e.g. listTools(t), callTool(t, name, args)), so the migration only has to swap that helper.

Today's generated schema nests arguments under a property named after the request message, while the handler reads them from the top level. Test what the handler does today (top-level arguments), and mark the schema mismatch with a // KNOWN BUG (#88/#89) comment instead of asserting the wrong schema.

Acceptance criteria

• go test ./... passes, covering all 5 VibeService RPCs plus at least one gRPC error case.
• Runs in under 5 seconds and needs no network or protoc.