[rust] Return error when table descriptor schema is missing.

[slfan1989]

Purpose

Linked issue: close #4173

TableDescriptorBuilder::build() returns a Result<TableDescriptor>, but currently panics when the required schema is not set.

This change makes the builder return Error::IllegalArgument instead, allowing callers to handle the invalid builder state through the existing Result API.

Brief change log

• Replace the expect() call in TableDescriptorBuilder::build() with an IllegalArgument error.
• Add a unit test verifying that building a table descriptor without a schema returns the expected error.
• Preserve the existing public method signature and successful build behavior.

Tests

• Added metadata::table::tests::table_descriptor_builder_requires_schema.

API and Format

No.

Documentation

No.

[fresh-borzoni]

@slfan1989 Thx, LGTM 👍