Add unit tests and GitHub Actions CI

[AdityaK011]

Summary

Adds 36 unit tests and GitHub Actions CI for the k8scope MCP server.

• Auth tests (24) — session CRUD, TTL expiry, one-time use codes, client registration, OAuth metadata, dynamic client
  registration, middleware auth gate, bearer extraction, rate limiting
• Tools tests (8) — input validation regex, helper functions (str, num), error formatting
• K8s tests (2) — cluster details cache hit and expiry
• CI workflow — build, vet, test with race detector on all pushes and PRs to main

Test plan

• go test ./... -v -race — 36/36 passing
• go vet ./... clean