GH-51361: [C++][Parquet] Derive records_to_read in FileReaderImpl::ReadColumn from RowGroup  (#51362)

### Rationale for this change
Fixes #51361.

### What changes are included in this PR?
Method `FileReaderImpl::ReadColumn` should derive `records_to_read` from the RowGroup rather than ColumnChunk's `num_values`. Deriving the right ColumnChunk index in `FileReaderImpl::DecodeRowGroups` is not trivial for nested schemas. This simplifies `FileReaderImpl::ReadColumn` and fixes #51361.

This was silently masked for full-schema reads and for columns with identical num_values(), but surfaces as a hard failure when an earlier, unselected column requires decryption: reading only a trailing plaintext column of a partially column-key-encrypted, plaintext-footer Parquet file threw "Cannot decrypt ColumnMetadata" even though the requested column was never encrypted.

This never corrupts data on unencrypted files: ReadColumn's wrong index is only ever used to look up ColumnChunk(i)->num_values(), a count fed into the *already-correct* reader as an upper bound on how many records to decode. Every row contributes at least one definition/repetition-level entry, so num_values() for any column is always >= that row group's true row count, and every column in a row group shares the same row count.

### Are these changes tested?
Yes, in the context of reading a plaintext column of a partially encrypted Parquet file. This cannot be tested with non-encrypted files.

### Are there any user-facing changes?
No.

### Was AI used for this PR?

In accordance to the [AI generation guidelines](https://arrow.apache.org/docs/dev/developers/overview.html#ai-generated-code), please disclose below whether and how AI was used in this PR.

**PR code and description written by:**

- [X] Human
- [X] AI

**Reviewed before submission by:**

- [X] Human
- [ ] AI
- [ ] Not reviewed
* GitHub Issue: #51361

Lead-authored-by: Enrico Minack <enrico.minack@insightsoftmax.com>
Co-authored-by: Enrico Minack <github@enrico.minack.dev>
Signed-off-by: Adam Reeve <adreeve@gmail.com>