feat(datafusion): make PartitionExpr reconstructible

[NoahKusaba]

Which issue does this PR close?

• Closes PartitionExpr cannot be reconstructed from its public API #3002.
• 1/5 of planned PR's that will close the Ballista-Iceberg integration.

What changes are included in this PR?

PartitionExpr wraps a PartitionValueCalculator, which is a live object with
no serializable form. Once the expression is built, nothing on it says which
partition spec and schema produced it, so a caller holding the expression
cannot rebuild an equivalent one.

Retain both inputs on the expression and expose them:

• try_new(partition_spec, table_schema) replaces new(calculator, spec) and
  builds the calculator itself, so the two inputs and the calculator cannot
  drift apart.
• partition_spec() and table_schema() read them back. Both are self-contained
  iceberg spec types, so a distributed engine can serialize them and rebuild
  the expression on a worker with try_new.

project_with_partition is unchanged from the outside; it now hands the two
inputs to try_new instead of constructing the calculator first.

Are these changes tested?

• test_partition_expr_rebuilds_from_its_retained_parts: reads partition_spec() and table_schema() off a PartitionExpr, builds a second one from them via try_new, and asserts both evaluate a RecordBatch to the same array.

The existing project tests cover the refactor itself, as it now constructs through try_new and still pass unchanged.

AI Disclosure

• Helped write the test.