test: add skipped UPDATE ... FROM coverage for issue #19950

[kosiew]

Which issue does this PR close?

• Part of UPDATE ...FROM bug #19950

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Rationale for this change

#20745 contains behavioral changes with a substantial amount of new test coverage to close #19950, but has more than 1000 lines, which makes review difficult as a single PR. This PR isolates the test additions into a tests-only precursor so reviewers can evaluate the expected behavior separately from the implementation.

The intent is to land the coverage scaffolding first without changing current behavior. The newly extracted tests are intentionally skipped and annotated with TODOs referencing #19950 so they can be enabled when the implementation PR lands.

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What changes are included in this PR?

This PR contains only test-side changes and the smallest compile-time scaffolding needed to carry those tests on this branch:

• Extracts the new/modified tests Support UPDATE ... FROM and preserve source-qualified assignments #20745

• Adds the extracted Rust tests to:

  • datafusion/core/tests/custom_sources_cases/dml_planning.rs
  • datafusion/core/src/physical_planner.rs test module
  • datafusion/sql/tests/sql_integration.rs

• Marks the extracted Rust tests as skipped with:

#[ignore = "TODO(19950): enable once the implementation PR lands"]

• Adds TODO(19950) comments to each skipped Rust test.

• Preserves the extracted SQLLogicTest coverage in:

datafusion/sqllogictest/test_files/update.slt

as commented-out skipped cases with TODO(19950) markers so the intended scenarios are present in the branch without being executed.

• Adds only minimal test-support scaffolding:

  • shared test schema/helpers used by the extracted tests
  • test-only helper functions in physical_planner.rs
  • a private helper signature adjustment in physical_planner.rs so the extracted planner tests compile against the current branch shape

No implementation fix for UPDATE ... FROM is included here.

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Are these changes tested?

Yes, the branch was verified to build with the newly added skipped tests in place using no-run test builds:

cargo test -p datafusion-sql --test sql_integration --no-run
cargo test -p datafusion --lib physical_planner::tests::test_create_not --no-run
cargo test -p datafusion --test core_integration --no-run
cargo test -p datafusion-sqllogictest --test sqllogictests --no-run

The newly extracted tests are intentionally skipped in this PR, so they compile but do not run until the implementation PR lands.

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Are there any user-facing changes?

No. This PR is tests-only in intent and does not change existing runtime behavior.

LLM-generated code disclosure

This PR includes LLM-generated code and comments. All LLM-generated content has been manually reviewed and tested.