test(yaad): split mcp_test.go into graph and concurrency/rest files

Mechanically split internal/server/mcp_test.go (1366 LOC) into three
same-package test files, moving tests verbatim with no behavior changes:

- mcp_test.go: shared test helpers plus remember/forget/pin/recall tests
- mcp_graph_test.go: link, subgraph, status, session, skill, feedback,
  context, and clamp/cancellation tests
- mcp_concurrency_rest_test.go: concurrency tests and REST API-key auth tests

Verified with go vet, go test, and golangci-lint (0 issues).

Author: Patel230