fix(core): clean up mcp event surface - #35221

Summary

• remove the legacy browser-open-failed MCP event from the current MCP event inventory/generated V2 SDK event union
• keep the named schema export available for legacy V1 callers
• add explicit messages for MCP typed errors that did not already carry one

Tests

• bun typecheck (packages/schema)
• bun typecheck (packages/core)
• bun typecheck (packages/sdk/js)
• bun test test/event-manifest.test.ts (packages/schema)
• bun test test/mcp.test.ts (packages/core)