fix: ignore constant aggregate completion boundaries

[yashrb24]

Which issue does this PR close?

• Closes A fixed GROUP BY column is incorrectly used to decide when groups are complete #25018

Rationale for this change

A grouping column with one fixed value never changes, so it cannot tell aggregation that an earlier group is complete.
Currently the logic uses such columns when choosing how aggregation processes its input. This can select the ordered path even when the remaining grouping columns are not ordered.

What changes are included in this PR?

• Ignore fixed grouping columns when deciding when groups can be completed.
• Preserve ordered aggregation when all grouping columns that can change are ordered.
• Update relevant plans

What is the testing strategy for this PR?

Added tests for a grouping column fixed by a filter. The tests fail without the fix.

Are there any user-facing changes?

No query results or public APIs change. DataFusion now chooses the appropriate aggregation path for fixed grouping columns.

[yashrb24]

Since #24697 is making the broader fix for this problem, we can possibly have it fixed in that PR instead of here

[codecov-commenter]

Codecov Report

❌ Patch coverage is 82.92683% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.90%. Comparing base (c2cf289) to head (41eda67).

Files with missing lines	Patch %	Lines
datafusion/physical-plan/src/aggregates/mod.rs	82.92%	2 Missing and 12 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25019      +/-   ##
==========================================
- Coverage   81.90%   81.90%   -0.01%     
==========================================
  Files        1134     1134              
  Lines      425261   425341      +80     
  Branches   425261   425341      +80     
==========================================
+ Hits       348325   348363      +38     
- Misses      56295    56320      +25     
- Partials    20641    20658      +17     

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[sunchao]

Thanks @yashrb24, LGTM.

Ignoring constant grouping keys for completion detection correctly rejects unbounded plans that cannot finish groups, while preserving incremental output when a varying grouping key is genuinely ordered.

Reviewed commit 037c8cf with five independent passes. Focused base/head validation passed 2,376 SQL cases per revision, eight paired streaming checks, and 40 low-memory cases per revision, including 16 spilling cases each. No actionable regressions found.

Regular and extended CI suites passed. The forced-hash-collision job timed out in unchanged COUNT DISTINCT capacity tests outside the modified aggregation code.