API: Fix FileRange validation order

[aviralgarg05]

Fixes #15922

Summary

Fix FileRange constructor validation so negative offset and length values are rejected before the fields are assigned.

Previously, the constructor validated length() and offset() before assigning the constructor arguments to the fields. That meant the checks were reading the default field values instead of the incoming arguments, allowing malformed ranges with negative values to be created.

Changes

• validate the constructor arguments directly in FileRange
• add regression tests covering negative length
• add regression tests covering negative offset

Why this matters

Invalid FileRange instances can propagate into vectored reads and fail later at runtime. Rejecting bad inputs at construction time keeps the API behavior correct and surfaces the error at the actual boundary where the bad state is introduced.

Testing

• ./gradlew :iceberg-api:test
• ./gradlew :iceberg-api:spotlessCheck