Add Vitest tests for Everything Server

Adds comprehensive test coverage for the Everything Server including:

Tools (10 tools tested):
- echo: message echoing with validation
- get-sum: number addition with edge cases
- get-env: environment variable retrieval
- get-tiny-image: image content blocks
- get-structured-content: weather data for all cities
- get-annotated-message: priority/audience annotations
- trigger-long-running-operation: progress notifications
- get-resource-links: dynamic resource link generation
- get-resource-reference: text/blob resource validation

Prompts (4 prompts tested):
- simple-prompt: no-argument prompt
- args-prompt: city/state arguments
- completable-prompt: department/name completions
- resource-prompt: embedded resource references

Resources:
- templates.ts: URI generation, text/blob resources
- session.ts: session-scoped resource registration

Test infrastructure:
- vitest.config.ts with v8 coverage
- Mock server helper for capturing registered handlers
- 81 tests, all passing

Closes #2925

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/everything/package.json

@@ -26,7 +26,8 @@
     "start:sse": "node dist/index.js sse",
     "start:streamableHttp": "node dist/index.js streamableHttp",
     "prettier:fix": "prettier --write .",
-    "prettier:check": "prettier --check ."
+    "prettier:check": "prettier --check .",
+    "test": "vitest run --coverage"
   },
   "dependencies": {
     "@modelcontextprotocol/sdk": "^1.26.0",
@@ -39,8 +40,10 @@
   "devDependencies": {
     "@types/cors": "^2.8.19",
     "@types/express": "^5.0.6",
+    "@vitest/coverage-v8": "^2.1.8",
     "shx": "^0.3.4",
     "typescript": "^5.6.2",
-    "prettier": "^2.8.8"
+    "prettier": "^2.8.8",
+    "vitest": "^2.1.8"
   }
 }