Core: Map struct key fields not accessible via short names

### Apache Iceberg version

None

### Query engine

None

### Please describe the bug 🐞

Schema.findField(String) returns null for short-form paths into map keys with struct types, even though the equivalent short-form paths into map values with struct types work correctly.

Have written a test in attached PR which show this and how to fix.

### Willingness to contribute

- [ ] I can contribute a fix for this bug independently
- [x] I would be willing to contribute a fix for this bug with guidance from the Iceberg community
- [ ] I cannot contribute a fix for this bug at this time

Start with Schema.findField(String) and compare how short-form paths are resolved for map values versus map keys containing structs. Review the test described in the issue or attached PR, then add coverage showing the key-path behavior and confirm the fix makes the test pass.

Written by the indexing model from the issue text.