feat: union nested Parquet leaves across projection accesses

[goutamadwant]

Which issue does this PR close?

• Closes Union the required leaves when a nested root is reached by multiple casts (or a cast plus get_field) instead of reading the whole column #24121.

Rationale for this change

When one projected nested column is consumed through multiple narrowing casts, or through both a narrowing cast and get_field, the Parquet read plan currently falls back to reading the entire root column. The leaves required by those consumers can be combined, so reading the full root performs unnecessary I/O.

What changes are included in this PR?

• Accumulate and deduplicate the leaf offsets required by every narrowing cast on a root column.
• Add leaves required by get_field accesses to the same union.
• Derive the Arrow type emitted by the reader from the merged leaf set while retaining a full-read fallback for unsupported partial wrapper shapes.
• Add coverage for disjoint, overlapping, and repeated casts, cast plus get_field, and casts that diverge below List<Struct>.

Are these changes tested?

Yes. The following checks pass:

• cargo fmt --all -- --check
• cargo test -p datafusion-datasource-parquet --lib
• cargo test --profile=ci --test sqllogictests -- parquet_nested_schema_pruning.slt
• cargo clippy -p datafusion-datasource-parquet --all-targets --all-features -- -D warnings

The SQL logic test also verifies that the merged projections return the expected values and scan fewer bytes than a full-root read.

Are there any user-facing changes?

No API or SQL behavior changes. Queries with multiple nested accesses to the same Parquet root can read fewer leaf columns.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.05556% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.06%. Comparing base (e0a05a6) to head (81c838a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ion/datasource-parquet/src/projection_read_plan.rs	92.20%	6 Missing ⚠️
...on/datasource-parquet/src/nested_schema_pruning.rs	94.02%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #24130    +/-   ##
========================================
  Coverage   81.06%   81.06%            
========================================
  Files        1107     1107            
  Lines      382191   382300   +109     
  Branches   382191   382300   +109     
========================================
+ Hits       309807   309904    +97     
- Misses      54082    54090     +8     
- Partials    18302    18306     +4     

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[kosiew]

@goutamadwant,

Thanks for working on this. The leaf union approach looks solid, and the added coverage for multiple casts, cast plus get_field, and nested list structs is helpful.

I only have one small non-blocking suggestion to make the fallback behavior a bit more explicit in the tests.

[kosiew]

Could we add a small unit test for the case where the union covers every leaf? type_for_leaf_subset intentionally returns None there so the caller falls back to the full root. It would be nice to pin that behavior explicitly and avoid a future change treating a full union as an unsupported partial projection.