Fix/test infra gameserver

[NAME-ASHWANIYADAV]

Draft — depends on #25 . Will be marked ready for review after those PRs merge and this branch is rebased.

Type of change

• 🐛 Bug fix
• 🚀 New feature / enhancement
• 📖 Documentation
• 🧹 Refactoring / cleanup
• ⚙️ CI / tooling
• ⚠️ Breaking change

What this PR does

Introduces Vitest testing infrastructure and adds 29 unit tests for the GameServer resource model.

Testing infra:

• vitest.config.ts - Configures Vitest with jsdom environment and aliases @kinvolk/headlamp-plugin/lib/k8s/cluster to a lightweight mock (the real path is externalized at build time and cannot be resolved in tests).
• src/__testutils__/kubeObjectMock.ts - Minimal KubeObject stand-in verified against the real Headlamp source.
• package.json - Updates the test script from headlamp-plugin test to vitest run.

GameServer tests (29 tests):

• Static fields, state, address, nodeName, fleet, ports, counters, lists, eviction, addresses, reservedUntil, mergedPorts.
• Edge cases: missing status, empty arrays, default values.

Did you use AI tools in preparing this PR?:
Y

Test plan

Automated checks

• npm ci
• npm run build
• npm run tsc
• npm run lint
• npm run format -- --check
• npm run test

Manual verification

N/A - test infrastructure and unit tests only.

Notes for reviewers

The KubeObject mock was verified against the real Headlamp source (this.jsonData = json, get metadata() { return this.jsonData.metadata }). The eviction getter defaults to 'Never', matching the upstream Agones Go code (EvictionSafeNever in gameserver.go:486).