Preserve recursive CTE static schema with plan-time schema alignment

[kosiew]

Which issue does this PR close?

• Closes Preserve recursive CTE declared schema when aligning physical children #22034.

Rationale for this change

RecursiveQueryExec widened recursive CTE output nullability by reconciling the static and recursive term schemas. This caused the physical schema to diverge from the logical/static CTE schema and forced valid SQL such as 0 AS level to be rewritten as nullable expressions like SUM(0) AS level.

This change preserves the declared recursive CTE schema by treating the static/anchor term schema as authoritative and aligning the recursive term to that schema during plan construction.

What changes are included in this PR?

• Added align_plan_to_schema, a higher-level plan-time schema alignment helper that guarantees the resulting plan advertises the expected schema exactly.

• Kept project_plan_to_schema as the narrower projection-based helper and refactored shared validation into validate_schema_alignment.

• Added SchemaAlignExec, an execution-plan adapter that:

  • advertises the expected schema from plan properties
  • preserves positional column values
  • rebinds emitted RecordBatch schemas inside the adapter
  • validates column count, data types, field metadata, and schema metadata

• Updated RecursiveQueryExec::try_new to:

  • use the static term schema as the recursive CTE output schema
  • align the recursive term with align_plan_to_schema
  • remove recursive output schema widening logic

• Restored the recursive CTE SLT coverage from SUM(0) AS level back to 0 AS level.

Are these changes tested?

Yes.

Added and updated tests covering:

• align_plan_to_schema:

  • exact schema returns unchanged plan
  • rename-only alignment uses ProjectionExec
  • nullable input to non-null expected schema uses SchemaAlignExec
  • column count mismatch errors
  • type mismatch errors
  • field metadata mismatch errors
  • schema metadata mismatch errors

• project_plan_to_schema:

  • schema match passthrough
  • nullability widening
  • nullability narrowing rejection
  • metadata mismatch validation

• RecursiveQueryExec:

  • recursive term projection alignment
  • preservation of the static nullability contract
  • recursive term schema matches the static schema after construction

• Restored SQL logic test coverage in cte.slt using 0 AS level.

Validated with:

cargo test -p datafusion-physical-plan recursive_query_exec
cargo test -p datafusion-physical-plan project_plan_to_schema
cargo test -p datafusion-sqllogictest --test sqllogictests -- cte

Are there any user-facing changes?

Yes.

Recursive CTEs now preserve the declared/static schema instead of widening nullability based on recursive expressions. Existing valid SQL such as:

0 AS level

continues to work without requiring nullable rewrites like:

SUM(0) AS level

LLM-generated code disclosure

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