refactor(unparser): centralize aggregate-scope rendering in the SQL unparser

[naman-modi]

Which issue does this PR close?

• Closes Centralize aggregate-scope expression rendering in the SQL unparser #23668.

Rationale for this change

• When printing a GROUP BY query, the unparser sometimes wraps the aggregate's input in an inner subquery (... FROM (SELECT ...)).
• Table aliases like cs only exist inside that subquery. Any clause outside it (SELECT, GROUP BY, HAVING, QUALIFY, ORDER BY) must use the subquery's output columns, not the aliases, so the unparser drops the alias.
• Each clause does that dropping on its own, so it is easy to miss one.
• ORDER BY on an aggregate that is not in the SELECT list was missed: it printed ORDER BY round(sum("cs"."total_revenue"), 2), while the SELECT list in the same query correctly printed sum("total_revenue").
• DataFusion reads that SQL back fine, but stricter databases reject it because cs is out of scope there.

What changes are included in this PR?

I moved the rule into one helper, UnparserAggScope, that checks once per aggregate whether the input is an inner subquery and then prepares expressions for each clause. SELECT, GROUP BY, HAVING, QUALIFY, and both ORDER BY paths now go through it, which fixes the ORDER BY case. The window-over-aggregate path is left as-is with a comment: it is only reachable from hand-built plans, since a window in SQL always sits inside a SELECT that already drops the alias. Only ORDER BY output changes; every already-correct case stays the same.

Are these changes tested?

Yes. New tests in datafusion/core/tests/sql/unparser.rs cover the inner-subquery shape for a window sorting by an aggregate, ORDER BY on an unselected aggregate (the fixed case), and a top-level ORDER BY (the second sort path). Existing unparser tests and the TPC-H/Clickbench roundtrips still pass.

Are there any user-facing changes?

ORDER BY over an aggregate is now printed without an out-of-scope table qualifier, so the generated SQL is valid for stricter databases. No API changes.

[naman-modi]

Hey @kosiew, can you please review this PR, when you get a chance?

One small design note: I went with a small UnparserAggScope struct that holds the aggregate and the "is the input a derived subquery" flag, computed once and reused by every clause, rather than a bare helper fn taking the flag as a parameter.

IMO, it reads a bit cleaner at the call sites and keeps the flag from being recomputed per clause. If you'd rather keep it consistent with the existing impl Unparser helpers, I'm happy to drop the struct and move it to a plain associated fn instead.

TIA!

[kosiew]

@naman-modi
Nice fix overall. I like the direction of consolidating the aggregate scope handling into a small helper. I just have a couple of minor suggestions that could make the abstraction a little cleaner and the test coverage a bit clearer.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.10638% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.65%. Comparing base (4a40101) to head (38cf748).

Files with missing lines	Patch %	Lines
datafusion/sql/src/unparser/plan.rs	85.10%	0 Missing and 7 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23789      +/-   ##
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- Coverage   80.65%   80.65%   -0.01%     
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  Files        1091     1091              
  Lines      370692   370702      +10     
  Branches   370692   370702      +10     
==========================================
+ Hits       298972   298974       +2     
+ Misses      53885    53884       -1     
- Partials    17835    17844       +9     

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[naman-modi]

Thanks for reviewing @kosiew! I've addressed both review comments. It would be great, if you could trigger the CI.