Partial updates keep the fields the client left out. Update/PATCH schemas that carried `.default()` re-applied those defaults under `.partial()`, so renaming a disabled reasoning-routing rule turned it back on, a playground preset lost its params and a proxy edit reset `family` to `auto`. The new `partialWithoutDefaults` helper strips defaults before `.partial()`; a scan test fails if any exported update schema ever leaks a default again.
Validated first on the combined board of all 38 PRs of this batch (10 merged as-is, 28 after the maintainer rework) on top of release/v3.8.51 c0f92ec: typecheck:core, check:open-sse-typecheck and check:dashboard-typecheck clean; ESLint clean on every changed file; file-size (rebaselined for the combined growth), complexity, cognitive-complexity, changelog-integrity, docs-counts, docs-sync, migration-numbering and i18n new-key gates green; 735 focused node:test cases with the only batch-caused failure (a flag-count assertion) fixed. Then re-validated alone on the fresh release tip right before this merge: ESLint on the changed files, typecheck:core, check:open-sse-typecheck, the file-size/complexity/changelog gates and this PR's own tests.
Thanks @maxmad64bis!
