test(mcp): replace module mocks with real servers

[rekram1-node]

Summary

• replace all process-global MCP test module mocks with real protocol fixtures
• exercise headers and lifecycle behavior through real Streamable HTTP and stdio MCP servers
• exercise OAuth discovery, registration, PKCE, token exchange, and authenticated reconnect against local endpoints
• add an injectable browser-launch boundary for deterministic OS integration coverage

Testing

• bun test test/mcp --only-failures (59 pass)
• bun typecheck
• bunx prettier --check src/mcp/index.ts src/mcp/browser.ts test/mcp/headers.test.ts test/mcp/lifecycle.test.ts test/mcp/oauth-auto-connect.test.ts test/mcp/oauth-browser.test.ts test/fixture/mcp-lifecycle-stdio.ts
• full package run: 3,100 pass; one unrelated existing Cerebras assertion fails independently