Fix minItems for all-optional array-like Structs - #1124

Problem

For an array_like=True Struct where every user field has a default, msgspec.json.schema reports minItems one too high. An untagged Struct gets minItems: 1 even though msgspec.json.decode accepts [] and fills all defaults; a tagged Struct similarly gets 2 even though a tag-only array is valid. Generated schemas therefore reject payloads accepted by msgspec itself.

The trailing-default loop stored the zero-based enumerate index and later used it as a count. When the loop never encountered a required field, that count was one short.

Fix

Count trailing optional fields explicitly before calculating minItems. The existing required-field and empty-Struct boundaries remain unchanged.

The regression test covers both tagged and untagged Structs and includes a default_factory, while checking the generated schema against the public decoder contract.

Testing

• just test tests/unit/test_schema.py — 124 passed, 2 skipped
• just test — 6412 passed, 113 skipped
• just check — lint, format, and spelling checks passed