refactor: centralize join-input table-scan filter extraction before u…

[Phoenix500526]

Which issue does this PR close?

• Closes Refactor: Centralize join-input normalization before SQL unparse recursion #23159

Rationale for this change

Join unparsing handles the left and right inputs inline in
select_to_sql_recursively (datafusion/sql/src/unparser/plan.rs). Both sides
repeated the same table-scan filter extraction
(try_transform_to_simple_table_scan_with_filters: extend the collected
filters, then return the simplified or original plan), while the
passthrough-Projection(Join) handling intentionally diverges per side (the
left input can be unwrapped; the right input may need a nested-join relation so
SQL preserves the required join grouping).

Centralizing only the duplicated common setup — and keeping the side-specific
behavior explicit — makes the two paths easier to read and keeps the
alias-scope invariant in one obvious place. #23159 tracks this.

What changes are included in this PR?

• Extract the duplicated filter-extraction dispatch into a small helper
  extract_join_input_table_scan_filters(plan, &mut table_scan_filters), and
  call it from both the left and right join-input paths.
• Keep the side-specific passthrough-projection handling inline and explicit
  (left: unwrap_qualified_passthrough_join_projection; right:
  qualified_passthrough_join_projection_to_nested_relation).
• Tests: factor the shared three-table setup into a
  nested_passthrough_join_tables() helper, and add
  test_unparse_projected_join_unwraps_left_nested_passthrough_projection to
  mirror the existing right-input test (this also fills a left-side coverage
  gap).

This is a pure refactor with no behavior change.

Are these changes tested?

Covered by existing tests plus the new left-input test, all passing:

• cargo test -p datafusion-sql --test sql_integration
  (includes both ..._left_..._passthrough_projection and
  ..._right_..._passthrough_projection)
• cargo clippy -p datafusion-sql --tests -- -D warnings
• cargo fmt --all

Existing inline snapshots are unchanged (no behavior change); the new test locks
in the left-input behavior the refactor touches.

Are there any user-facing changes?

No. This is an internal refactor of the SQL unparser; the generated SQL is
unchanged and there are no public API changes.

[kosiew]

@Phoenix500526
Thanks for working on this.

Looks 👍 to me