Reuse the JSON object checks before filtering MCP merge payloads so ordinary
JSON merges retain their existing behavior when ECC_DISABLED_MCPS is set.
Add 10 regression cases covering payload shapes, preview and install state,
file contents, and plan immutability. Nine cases reproduce the original
failure. All 224 cases across 10 installer/lifecycle suites pass.
Validation: changed-file ESLint (no errors), git diff --check, and apply.js
coverage of 91.05% lines, 83.70% branches, and 100% functions.