fix(config): preserve valid fields during migration

[luoye520ww]

Summary

Fix GUI-managed Kun config sanitization so one stale or unknown option cannot discard an otherwise valid config section. This closes the remaining failure mode behind #886 on older config files.

Scope

This PR is intentionally limited to preserving known valid fields while removing legacy fields. It is not a full schema-version migration; that remains a separate follow-up with its own backup, migration, and downgrade tests.

Changes

• Retry strict section parsing with the schema's known keys when an old top-level option is present.
• Validate known nested values independently so an obsolete nested option does not erase unrelated runtime settings.
• Keep invalid known values rejected; no secrets are newly persisted by this change.
• Add a regression assertion that a valid runtime token survives sanitization while the legacy key is removed.

Tests

npm.cmd exec vitest run src/main/kun-process.test.ts
npm.cmd run typecheck
npm.cmd run lint -- --quiet
npm.cmd run build
git diff --check

Results: 45 kun-process tests passed; Typecheck, lint, build, and diff check passed locally.

CI note

The previous GitHub run executed 55 tests successfully, then failed in the extension release gate while downloading the Electron binary (TypeError: fetch failed / Electron failed to install correctly). No changed config code was involved. The next synchronized run should be used to revalidate the external download step.

Issue

Part of #886

[himetuki]

加油💪

[XingYu-Zhong]

Reviewed against current develop after branch update. Scope is focused, the change remains needed, and no blocking issue was found. Required CI must pass before merge.