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 81.04%. Comparing base (f27e50c) to head (943a24a).

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   81.04%   81.04%            
========================================
  Files        1105     1105            
  Lines      380092   380201   +109     
  Branches   380092   380201   +109     
========================================
+ Hits       308041   308139    +98     
- Misses      53835    53840     +5     
- Partials    18216    18222     +6     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.