feat(catalog): expose InformationSchemataBuilder as public API

[zfarrell]

Which issue does this PR close?

No linked issue. Happy to file one if reviewers prefer.

Rationale for this change

Downstream catalog implementations that resolve schemas asynchronously
cannot reuse InformationSchemaProvider — it enumerates schemas via
CatalogProvider::schema_names(), which is synchronous, so an
async-only catalog has to provide its own information_schema.schemata
view. Today that requires either duplicating the column layout and
the row-building logic, or reaching into private items.

Exposing InformationSchemataBuilder and a schemata_schema() factory
lets external crates emit byte-for-byte-compatible schemata batches
without copy-pasting the contract.

What changes are included in this PR?

• pub fn schemata_schema() -> SchemaRef — extracts the column-layout
  factory. InformationSchemata::new now calls it instead of inlining
  the schema, so there is a single source of truth.
• InformationSchemataBuilder becomes pub (was private) with a
  Default impl and a public new(). add_schemata and finish are
  bumped to pub. The function bodies and parameter types
  (&str / Option<&str>) are unchanged.
• finish now returns Result<RecordBatch> instead of panicking via
  an internal .unwrap(). The one internal caller
  (PartitionStream::execute for InformationSchemata) was previously
  wrapping Ok(builder.finish()) and is updated to just
  builder.finish() since the inner expression now produces the
  Result directly.

Are these changes tested?

Yes. A new unit test schemata_builder_emits_canonical_schema_and_rows
exercises the public API end-to-end via Default::default(), asserts
the produced batch's schema matches schemata_schema(), and verifies
the null pattern for schema_owner, the three default_character_set_*
columns, and sql_path. The pre-existing internal users
(InformationSchemata::new, PartitionStream::execute) continue to
exercise the same code path through the unchanged
InformationSchemata::builder() constructor.

Are there any user-facing changes?

Yes — three new public items in datafusion-catalog: schemata_schema,
InformationSchemataBuilder (with its new / add_schemata / finish
methods + Default impl). No existing public API is broken. The
Result<RecordBatch> return on finish is a first-time-public surface,
not a regression.

[alamb]

Makes sense t0 me -- thank you @zfarrell