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>

package-lock.json

@@ -3806,9 +3806,11 @@
       "devDependencies": {
         "@types/cors": "^2.8.19",
         "@types/express": "^5.0.6",
+        "@vitest/coverage-v8": "^2.1.8",
         "prettier": "^2.8.8",
         "shx": "^0.3.4",
-        "typescript": "^5.6.2"
+        "typescript": "^5.6.2",
+        "vitest": "^2.1.8"
       }
     },
     "src/filesystem": {