test: cover oneOf/enum/not null branches; harden and dedup validation
Address adversarial review feedback:
- Add tests for the previously untested null-composition branches of
schemaAllowsNullWithoutNullable (oneOf exactly-one, enum null
containment, not) plus oneOf null rejection/acceptance through the
public ValidateInputMapForMode entry point, protecting preflight vs
runtime parity.
- Dedup the two identical quoting loops and four-way message branch in
validateKnownInputs into a quoteJoin helper.
- Harden missingRequiredInput to key off schemaErr.SchemaField
('required') before parsing the free-text reason.
- Document validation ownership across the three entry points and add a
defensive nil guard in NewInputsForMode.
cog
eb8cd646 - test: cover oneOf/enum/not null branches; harden and dedup validation
