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/vitest.config.ts

@@ -0,0 +1,14 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: 'node',
+    include: ['**/__tests__/**/*.test.ts'],
+    coverage: {
+      provider: 'v8',
+      include: ['**/*.ts'],
+      exclude: ['**/__tests__/**', '**/dist/**'],
+    },
+  },
+});