feat!: generate MCP servers on the official Go MCP SDK #89

Closes #44. The core migration from github.com/mark3labs/mcp-go to the official SDK, github.com/modelcontextprotocol/go-sdk v1.8.0 (requires Go 1.25). It lands on main; the old official-go-mcp branch and draft #74 are superseded, but #74's typed-handler idea is fine to reuse.

Do

• go.mod: go 1.25, require github.com/modelcontextprotocol/go-sdk v1.8.0. Remove mark3labs from go.mod and go.sum.
• Generator (cmd/protoc-gen-go-mcp/mcp.go), for each non-streaming RPC:
  • Tool: &mcp.Tool{Name, Description, InputSchema: <schema from #88 for the request message, emitted as a json.RawMessage literal>}.
  • Registration: mcp.AddTool[json.RawMessage, any](server, tool, handler) so that the SDK validates arguments against the schema (raw Server.AddTool does not validate).
  • Handler: protojson.Unmarshal(args, req) → call the gRPC client.
    • On success: return &mcp.CallToolResult{Content: []mcp.Content{&mcp.TextContent{Text: protojson.Marshal(resp)}}}.
    • On a gRPC error: return it as a Go error, which the SDK turns into an isError result. Include the gRPC status code in the message.
  • Exported API: keep the generated names and shape (New<Service>MCPServer, the per-tool accessors, RegisterDefaultTools) wherever the official types allow, and list any change in the PR body.
• Examples: regenerate them, and update the golden file (test: golden-file test for the plugin's generated code #85).
• cmd/mcp-vibe: mcp.NewServer(&mcp.Implementation{...}, nil) + server.Run(ctx, &mcp.StdioTransport{}).
• End-to-end test (test: end-to-end test of a generated MCP server (gRPC backend + MCP client) #87): swap its MCP-client helper to mcp.NewInMemoryTransports() + mcp.NewClient(...).Connect(...) + ClientSession.CallTool.
  • Resolve the KNOWN BUG markers: the advertised schema must now be the top-level request schema.
  • Add a case where invalid arguments (wrong type or unknown field) produce isError: true and never reach the backend.
• PR: a feat!: title and a BREAKING CHANGE: note in the body listing what users must change.

Acceptance criteria

• grep -rn mark3labs . finds nothing (outside the CHANGELOG).
• go build ./..., go vet ./... and go test ./... pass, including the golden and e2e tests.
• Regeneration: make generate reproduces the committed examples exactly.
• Smoke test, pasted in the PR: mcp-vibe over stdio answers initialize, tools/list and one tools/call.