Expose negotiated protocol version in v2

[shusingh]

Summary

• expose the negotiated MCP protocolVersion through InspectorClient and useInspectorClient
• update the v2 app header data to use the negotiated version instead of a hard-coded value
• add hook and integration coverage for protocol version propagation

Fixes #1324.

Validation

• node node_modules/vitest/vitest.mjs run --project=unit src/test/core/react/useInspectorClient.test.tsx
• node node_modules/vitest/vitest.mjs run --project=integration src/test/integration/mcp/inspectorClient.test.ts -t "captures the negotiated protocol version"
• node node_modules/typescript/bin/tsc -b
• node node_modules/vite/bin/vite.js build
• node node_modules/prettier/bin/prettier.cjs --check <touched files>
• node node_modules/eslint/bin/eslint.js .
• git diff --check