fix: preserve statistics for safe casts without extrema

[viirya]

Which issue does this PR close?

N/A

Rationale for this change

Projection statistics currently infer a CAST's source type from its minimum or
maximum value. When both extrema are absent, even a lossless widening cast such
as Int32 to Int64 is treated as unsafe.

This discards otherwise valid statistics, including exact null counts and
distinct counts, and can reduce the quality of optimizer estimates.

What changes are included in this PR?

• Add ProjectionExprs::project_statistics_with_input_schema, which uses the
  input schema to identify lossless casts when typed extrema are unavailable.
• Preserve the existing conservative behavior for casts that may fail, discard
  values, or change ordering.
• Use the schema-aware method in ProjectionExec, extended statistics
  projection, and file scan statistics projection.
• Add helper-level and ProjectionExec regression tests covering an Int32 to
  Int64 cast with absent extrema but exact null and distinct counts.

The existing project_statistics API remains available, so this is an
additive, non-breaking API change.

What is the testing strategy for this PR?

Added:

• test_project_statistics_safe_cast_without_extrema
• test_projection_statistics_safe_cast_without_extrema

Ablation testing confirmed that the old implementation changes
null_count = Exact(3) to Absent. Both the helper-level and production
ProjectionExec paths preserve the exact null and distinct counts after the
fix.

Also verified:

• Targeted regression tests
• cargo fmt --all -- --check
• git diff --check

The full test suite is covered by CI.

Are there any user-facing changes?

Optimizer statistics are now preserved through known-lossless casts even when
minimum and maximum statistics are unavailable.

This PR adds a public method but does not remove or change the existing API.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.07843% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.42%. Comparing base (710c748) to head (64aa719).
⚠️ Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/datasource/src/file_scan_config/mod.rs	80.00%	2 Missing ⚠️
...usion/physical-plan/src/operator_statistics/mod.rs	80.00%	1 Missing ⚠️
datafusion/physical-plan/src/projection.rs	96.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25517      +/-   ##
==========================================
+ Coverage   82.38%   82.42%   +0.03%     
==========================================
  Files        1138     1138              
  Lines      434313   435405    +1092     
  Branches   434313   435405    +1092     
==========================================
+ Hits       357805   358868    +1063     
+ Misses      54876    54833      -43     
- Partials    21632    21704      +72     

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[alamb]

@haohuaijin can you help review this PR?

[alamb]

Thanks @viirya -- would it be possible to get a end to end test for this fix? Specifically create a parquet file and then review the stats that come out. As an example, I think @haohuaijin did this in #25227

[haohuaijin]

Thanks @viirya , LGTM!

I reproduced the null-count loss on main use below sql(because memtable only have null count, do not have min/max):

SET datafusion.explain.format = 'indent';
SET datafusion.explain.show_statistics = true;

CREATE TABLE t(a INT) AS VALUES (1), (NULL);
EXPLAIN SELECT CAST(a AS BIGINT) FROM t;

The scan reports Null=Exact(1), but the CAST projection drops it. This PR preserves that statistic.

[alamb]

Thanks for the review @haohuaijin

[viirya]

Thanks for the suggestion! I added an end-to-end regression test based on the reproducer and verified it fails without the fix and passes with it.