fix: scope MCP uninstall cleanup to managed targets

[Aryan Singh K. (aryansk)]

Summary

Make uninstall MCP cleanup honor the target ownership recorded in the lockfile instead of sweeping every runtime, tolerate JSONC comments in JetBrains Copilot config, and continue cleanup across target-specific write failures before reporting an error.

Changes

• restrict stale MCP removal to runtimes that actually own each server in mcp_target_servers
• run target cleanup independently so one broken config does not leave unrelated managed targets orphaned
• parse JetBrains Copilot JSONC safely without treating // inside strings as comments
• add regressions for target scoping, JSONC URLs, and partial cleanup failure

Validation

Prepared focused regression tests:
pytest -q tests/unit/regressions/test_issue_2551_mcp_uninstall_cleanup.py

Patch structure was validated with git apply --check. Full repository tests should run in CI.

Fixes #2551