refactor: cache schema_without_virtual_columns and remove TableSchema::with_virtual_columns

[mbutrovich]

Which issue does this PR close?

Followup to #22026 — addresses two review comments from @adriangb that landed after the PR was already in the merge queue.

Rationale for this change

Two pieces of feedback on the TableSchema API introduced in #22026:

1. feat: Plumb Parquet virtual columns (row_number) through TableSchema and ParquetOpener #22026 (comment) — TableSchema::with_virtual_columns shouldn't exist as a non-deprecated counterpart to the already-deprecated TableSchema::with_table_partition_cols. Callers should use TableSchemaBuilder directly. Every existing call site already does, so the method has no users.
2. feat: Plumb Parquet virtual columns (row_number) through TableSchema and ParquetOpener #22026 (comment) — schema_without_virtual_columns was rebuilding the schema on every call. It can be computed once at construction and returned by reference, matching the convention used by every other accessor on TableSchema (file_schema, table_schema, table_partition_cols, virtual_columns all return &).

What changes are included in this PR?

• Remove TableSchema::with_virtual_columns. Callers must use TableSchemaBuilder::with_virtual_columns (no in-tree callers needed updating).
• Cache schema_without_virtual_columns on TableSchema, computed once in TableSchemaBuilder::build. When there are no virtual columns the cached field shares the same Arc as table_schema.
• Accessor schema_without_virtual_columns(&self) now returns &SchemaRef instead of an owned SchemaRef, matching the rest of the struct's accessors. Updated the one in-tree caller in datasource-parquet/src/source.rs.

Are these changes tested?

Yes — covered by the existing table_schema unit tests and the datasource-parquet test suite, all of which still pass. The change is a refactor with no behavioral difference: the cached schema produced in build() is byte-for-byte identical to what the previous accessor allocated on each call.

Are there any user-facing changes?

Two API changes against the TableSchema surface added in #22026 (which has not been released):

• TableSchema::with_virtual_columns removed. Use TableSchemaBuilder::with_virtual_columns instead.
• TableSchema::schema_without_virtual_columns return type changed from SchemaRef to &SchemaRef. Callers that need an owned value should Arc::clone the result.

[github-actions]

Thank you for opening this pull request!

Reviewer note: cargo-semver-checks reported the current version number is not SemVer-compatible with the changes in this pull request (compared against the base branch).

Details

     Cloning apache/main
    Building datafusion-datasource v53.1.0 (current)
       Built [  40.309s] (current)
     Parsing datafusion-datasource v53.1.0 (current)
      Parsed [   0.035s] (current)
    Building datafusion-datasource v53.1.0 (baseline)
       Built [  35.680s] (baseline)
     Parsing datafusion-datasource v53.1.0 (baseline)
      Parsed [   0.035s] (baseline)
    Checking datafusion-datasource v53.1.0 -> v53.1.0 (no change; assume patch)
     Checked [   0.340s] 222 checks: 221 pass, 1 fail, 0 warn, 30 skip

--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.47.0/src/lints/inherent_method_missing.ron

Failed in:
  TableSchema::with_virtual_columns, previously in file /home/runner/work/datafusion/datafusion/target/semver-checks/git-apache_main/3cf7effc87b9920734a7bd585f7256ccb157c4b2/datafusion/datasource/src/table_schema.rs:184
  TableSchema::with_virtual_columns, previously in file /home/runner/work/datafusion/datafusion/target/semver-checks/git-apache_main/3cf7effc87b9920734a7bd585f7256ccb157c4b2/datafusion/datasource/src/table_schema.rs:184

     Summary semver requires new major version: 1 major and 0 minor checks failed
    Finished [  78.701s] datafusion-datasource
    Building datafusion-datasource-parquet v53.1.0 (current)
       Built [  41.557s] (current)
     Parsing datafusion-datasource-parquet v53.1.0 (current)
      Parsed [   0.030s] (current)
    Building datafusion-datasource-parquet v53.1.0 (baseline)
       Built [  41.340s] (baseline)
     Parsing datafusion-datasource-parquet v53.1.0 (baseline)
      Parsed [   0.031s] (baseline)
    Checking datafusion-datasource-parquet v53.1.0 -> v53.1.0 (no change; assume patch)
     Checked [   0.193s] 222 checks: 222 pass, 30 skip
     Summary no semver update required
    Finished [  84.655s] datafusion-datasource-parquet

[adriangb]

Thanks!