feat: Restore nullability when consuming substrait fields

[neilconway]

Which issue does this PR close?

• Closes Restore nullability support for consuming Substrait fields. #12727.

Rationale for this change

The Substrait logical-plan consumer was discarding field nullability when reconstructing DataFusion schemas from Substrait struct types. Nullability matters because a Substrait plan may have been produced or optimized using non-null guarantees.

This also improves DataFusion <-> Substrait round-trip fidelity: required fields encoded by the producer are preserved when the plan is consumed again, instead of being widened to nullable.

What changes are included in this PR?

• Preserve per-field nullability when converting Substrait struct types / NamedStruct schemas into DataFusion schemas.
• Treat Substrait Required as non-nullable, and Nullable, Unspecified, or unknown nullability values as nullable.
• Keep deprecated UserDefinedTypeReference non-nullable because it does not carry nullability metadata.
• Enforce named-table ReadRel schema compatibility when the Substrait schema requires a field to be non-null but the resolved DataFusion table schema marks it nullable.
• Extend compatibility checking recursively through nested Struct fields.
• Leave List and Map child nullability compatibility as future work, since their child nullability is not faithfully reconstructed today.

Are these changes tested?

Yes; new tests added.

Are there any user-facing changes?

We are a bit stricter when consuming Substrait plans now, but that could prevent problems: for example, if a Substrait plan was produced under the assumption that a field x is non-nullable but the local DataFusion schema allows null values in x, executing the plan might produce unexpected results.

[neilconway]

Initially motivated by the test failure in #22089, but seems worth improving as a separate task rather than just weakening the test assertions.