feat: feature-gate datafusion-substrait behind optional 'substrait' feature

[zhuqi-lucas]

Which issue does this PR close?

Closes #21269

Rationale for this change

datafusion-substrait is a heavyweight dependency that most developers and users don't need. Making it optional saves compile time for the common case.

What changes are included in this PR?

• datafusion-substrait is now an optional dependency in datafusion-sqllogictest/Cargo.toml
• New substrait feature flag gates the dependency
• #[cfg(feature = "substrait")] gates the module, imports, and run_test_file_substrait_round_trip function
• No-op fallback function returns an error when the feature is disabled
• CI workflow updated to pass --features substrait for the round-trip test

Are these changes tested?

• Compiles without substrait feature (default)
• Compiles with --features substrait
• CI substrait round-trip test updated to enable the feature

Are there any user-facing changes?

No — this only affects the sqllogictest binary, not the datafusion library itself. The substrait round-trip test requires --features substrait to run.

[Copilot]

Pull request overview

This PR makes the datafusion-substrait integration in the datafusion-sqllogictest crate opt-in behind a new substrait feature flag to reduce compile time and dependency weight for the default build.

Changes:

• Make datafusion-substrait an optional dependency and add a substrait feature in datafusion/sqllogictest/Cargo.toml.
• Gate the Substrait round-trip engine module and public re-exports behind #[cfg(feature = "substrait")].
• Add a non-substrait fallback for the Substrait round-trip runner and update CI to enable the feature when running that mode.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
datafusion/sqllogictest/src/lib.rs	Gate the public re-export of DataFusionSubstraitRoundTrip behind the substrait feature.
datafusion/sqllogictest/src/engines/mod.rs	Gate the Substrait round-trip engine module and re-export behind the substrait feature.
datafusion/sqllogictest/Cargo.toml	Make datafusion-substrait optional and introduce a substrait feature enabling it.
datafusion/sqllogictest/bin/sqllogictests.rs	Conditionally import/use the Substrait engine; provide a runtime error fallback when feature is disabled.
.github/workflows/rust.yml	Update Substrait round-trip CI job to pass --features substrait.

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[Copilot]

The cargo test --test sqllogictests --features substrait ... invocation is run from the workspace root in a virtual workspace. Adding --features substrait here is likely to fail because features must be applied to a specific package (e.g. --package datafusion-sqllogictest) or by running the command from datafusion/sqllogictest/. Consider updating this step to either cd datafusion/sqllogictest before running, or add --package datafusion-sqllogictest (and optionally --profile ci to match other CI invocations).

Suggested change

	run: cargo test --test sqllogictests --features substrait -- --substrait-round-trip limit.slt
	run: cargo test -p datafusion-sqllogictest --test sqllogictests --features substrait -- --substrait-round-trip limit.slt

[zhuqi-lucas]

CI failure in verify benchmark results is a pre-existing issue on main — explain.slt JSON format mismatch, unrelated to this PR (we only changed substrait feature gating).