perf: embed projections in ASOF joins

[Xuanwo]

Which issue does this PR close?

• Part of ASOF join support / Specialize Range Joins #318.
• Umbrella PR: feat: support ASOF joins #23738.
• Follow-up to feat: add ASOF join physical operator #23828.

Rationale for this change

AsOfJoinExec already supports an internal output projection, but physical projection pushdown cannot currently populate it. Queries that need only a subset of ASOF output columns therefore retain a separate ProjectionExec and materialize unused join columns.

What changes are included in this PR?

• Implement the physical projection-embedding hook for AsOfJoinExec.
• Preserve empty-projection row counts and decline unsafe repeated embedding.
• Add focused execution coverage for projected, empty, and already-projected plans.

Are these changes tested?

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test -p datafusion-physical-plan joins::asof_join::tests --all-features
• Extended workspace tests from the contributor guide

Are there any user-facing changes?

No SQL or logical API changes. Physical plans may avoid materializing ASOF output columns that are not required downstream.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.65979% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.62%. Comparing base (3a4c310) to head (63d9434).

Files with missing lines	Patch %	Lines
datafusion/physical-plan/src/joins/asof_join.rs	88.65%	2 Missing and 9 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #24873   +/-   ##
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  Coverage   81.62%   81.62%           
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  Files        1123     1123           
  Lines      409637   409734   +97     
  Branches   409637   409734   +97     
=======================================
+ Hits       334383   334465   +82     
- Misses      55624    55629    +5     
- Partials    19630    19640   +10     

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