Document and add regression tests for safeColumns in ParquetReadOptions

[Anamika1608]

ParquetReadOptions currently exposes a safeColumns :: Bool option in the Parquet reader API, but it does not seem to be covered in the user-facing
Parquet documentation or the Parquet test suite.

Current state in main:

• safeColumns is part of ParquetReadOptions
• it is implemented in the Parquet read path
• but the cookbook section for Parquet options only documents:
  • selectedColumns
  • predicate
  • rowRange
• I also could not find a regression test specifically covering safeColumns

[Anamika1608]

I would like to take this up, if this scope looks useful to maintainers.

Planned approach:

• document safeColumns alongside the other Parquet read options
• add a regression test for the current behavior
• keep the PR limited to docs + tests, without changing semantics