fix(agent): skeletonFromJsonSchema handles nullable type-union shape

[ygd58]

Fixes #614

exampleFromJsonSchema (via its internal skeletonFromJsonSchema helper) returned null for any field whose JSON Schema type is an array containing "null" (e.g. { type: ["string", "null"] }), instead of a type-appropriate placeholder. The switch(schema.type) statement never matched an array, so it always fell through to the null default.

The type field can validly be an array of alternatives per the JSON Schema spec, and some Zod versions (and non-Zod JSON Schema producers) represent a nullable field this way rather than as anyOf. exampleFromJsonSchema is a public entry point for schemas generated by whatever tooling the caller uses (per its own doc comment), so this shape must be handled regardless of which zod version/tool produced it - not just the anyOf branch already handled above.

Fix: normalize an array type to its non-null member before the switch.

Tests: this repo's bundled zod version actually produces anyOf for .nullable() (not a type array), so the zod-based tests pass via the pre-existing anyOf handling and mainly guard against regressions there. The raw type-array tests (constructing { type: [...] } directly) exercise the actual fix and fail without it - verified by temporarily reverting the fix and confirming that test failed, then restoring it.

Changeset: added (@sapiom/agent, patch).

Verified locally: vitest run src/introspection.test.ts (5/5 passing), eslint clean.