Null-safe value handling

Make null/undefined (and “invalid/unknown”) states explicit and prevent them from flowing into non-null/non-optional operations.

Apply these rules: 1) Enforce compiler null safety: enable strictNullChecks so the typechecker blocks x?.foo()/x.foo() on string | null-like values. 2) Guard before method calls: if a value can be null/undefined, check it before calling string/array methods. 3) Align conversion boundaries: parsing/normalization that accepts unknown must either:

• return a valid, strongly-typed value, or
• fail fast (throw) / return a discriminated union that forces handling. Never “parse may return undefined” but then pass the result into a function typed as “number only”. 4) Make fallback logic unambiguous: when using ?? default, ensure undefined (missing support) and false (explicitly disabled) are treated differently.

Example (null guard + explicit fallback):

function getErrorPrefix(err: string | null | undefined) {
  if (!err) return null; // handles ''/null/undefined per your policy
  return err.startsWith('No API key found') ? err : null;
}

const constrainedStrict = supportsStrictMode
  ? resolveJsonSchemaStrictSampling(tool, true)
  : undefined; // explicit
const strict = constrainedStrict ?? defaultStrict; // only applies when constrainedStrict is undefined

This prevents unsafe member access, accidental propagation of undefined/invalid values across typed boundaries, and confusing fallback behavior.