fix: resolve pass-through columns against the input when merging an extraction projection - #25412

Which issue does this PR close?

None filed.

Rationale for this change

This query fails to plan on main:

create table t(v int, s struct<a int>, env varchar) as values (1, {a: 10}, 'prod'), (2, {a: 20}, 'dev');

with samples as (
  select v, s, env from t
),
expanded as (
  select v, s, env from samples
  union all
  select v, s, env from samples where 1 = 2
)
select env, sum(s['a']) from expanded group by env;

Optimizer rule 'push_down_leaf_projections' failed
caused by
Schema error: Schema contains qualified field name samples.env and unqualified
field name env which would be ambiguous

enable_leaf_expression_pushdown is on by default, so this is a plain planning
failure for a valid statement. A union all whose one side is provably empty is
what produces the shape, and query generators emit that: a dashboard panel adds a
second branch behind a comparison of two constants and the panel stops working.

What changes are included in this PR?

build_extraction_projection_impl merges an extraction projection into the
projection below it, and then adds the pass-through columns the merged projection
does not already carry. It compared the columns it was about to add against the
projection's own expressions without putting the two in the same space.

A projection can list bare column names over a qualified input. Removing the empty
side of the union leaves exactly that. The comparison then misses, the column is
added a second time under its bare name, and Projection::try_new rejects a schema
that holds samples.env and a bare env together.

This PR resolves both sides against the input schema before the comparison, and
pushes the column under the name the input gives it. A name the input schema holds
more than once resolves to nothing, because no single spelling is correct there.
A same-name alias such as t.c AS c counts as a pass-through on the existing side too, the same as a bare t.c.

Are these changes tested?

Yes. The statement above is added to struct.slt. It fails on main with the
error above and passes with this change.

Two unit tests in extract_leaf_expressions.rs cover the merge directly. Both fail on main with an ambiguous test.a error:

• test_merge_bare_column_into_qualified_projection: a filter names a over a projection that outputs test.a.
• test_merge_into_projection_with_same_name_alias: the projection spells the pass-through as test.a AS a.

cargo test --workspace --exclude datafusion-sqllogictest passes: 126 suites,
12479 tests, 0 failures. The sqllogictest suite passes too: 520 files, 0
failures. cargo clippy -p datafusion-optimizer --all-targets and
cargo fmt --all -- --check are clean.

Are there any user-facing changes?

A statement of this shape plans instead of failing. No API change.

Note on overlapping work

#25388 (draft) edits the same arm of
build_extraction_projection_impl, for a different problem: duplicated
evaluation of KeepInPlace expressions, #25329.
It does not touch the qualifier comparison this PR changes, so the two are
independent in behaviour, but whichever lands second will need a small rebase at
the tail of the columns_needed loop.

#25282 also appends to the end of
struct.slt, as this PR does. Keeping both blocks is the whole resolution.

Part of the leaf-pushdown EPIC: #25459