perf: mirror ASOF equality-key filters to right

[Xuanwo]

Which issue does this PR close?

• Part of ASOF join support / Specialize Range Joins #318.
• Follow-up to perf: push left filters through ASOF joins #24801.

Rationale for this change

When a filter selects one ASOF equality-key group on the left, rows in other right-side groups cannot match any surviving left row. The broadcast join still sorts and retains those right rows unless the same key filter is applied to the right input.

Performance

Q09 has 1M rows on each side across 10K equality groups. WHERE l.key = 42 retains one group and produces 100 rows with both revisions. On an Apple M4 Max, using release builds and two 7-iteration runs per revision, the median elapsed time across all 14 iterations was:

Revision	Median elapsed
main without this optimization (b172f73f0)	21.5 ms
This change (b3fd5f1f9)	2.8 ms

This selective keyed workload is about 7.8x faster (87% lower elapsed time). The benchmark file and command were the same for both binaries:

target/release/benchmark_runner asof_join --query 9 --iterations 7

The branch was subsequently synced through 21a3215b6; those upstream-only commits do not touch the optimizer or ASOF paths exercised by Q09.

What changes are included in this PR?

• Mirror a pushed left_key = literal predicate to the corresponding right key when both join keys are direct columns of the same type.
• Keep the original left filter and ASOF null-padding behavior unchanged. Other predicate shapes and coerced or expression-based keys are not inferred.
• Add result and plan coverage in asof_join.slt, plus a keyed selective workload as ASOF benchmark Q09.

What is the testing strategy for this PR?

The SLT cases check the mirrored plan, matching and unmatched results, a right-side IS NULL filter, and a left disjunction that must not prune the right input. Q09 runs the user-visible query with 1M rows per side across 10K groups, selecting one group.

Are there any user-facing changes?

No API or result changes. Eligible ASOF joins can sort and retain fewer right-side rows.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 74.07407% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.50%. Comparing base (21a3215) to head (bf38b31).

Files with missing lines	Patch %	Lines
datafusion/optimizer/src/push_down_filter.rs	74.07%	4 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #25574   +/-   ##
=======================================
  Coverage   82.50%   82.50%           
=======================================
  Files        1140     1140           
  Lines      438050   438077   +27     
  Branches   438050   438077   +27     
=======================================
+ Hits       361410   361434   +24     
+ Misses      54833    54830    -3     
- Partials    21807    21813    +6     

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[Xuanwo]

Hi @jayzhan211 and @2010YOUY01, this follow-up is now restacked and ready for review. Would you mind taking a look when you have time? Thanks!