datafusion/substrait: enable unicode_expressions in dev-dependencies to fix substring planning test

[kosiew]

Which issue does this PR close?

• Closes Substring not planned under Substrait when unicode_expressions` feature is disabled #17582

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Rationale for this change

A test failure was observed when building the DataFusion Substrait integration with only the nested_expressions feature enabled in datafusion/substrait/Cargo.toml. The UnicodeFunctionPlanner was not registered when unicode_expressions was disabled, causing the SUBSTR / substring function to be unplannable during Substrait logical plan conversion. This produced the error shown in the test:

NotImplemented("Substring could not be planned by registered expr planner. Hint: enable the unicode_expressions")

This PR updates the dev-dependency for datafusion within datafusion/substrait/Cargo.toml to include the unicode_expressions feature so that the simple_scalar_function_substr test can run reliably and the Unicode function planner is available in test builds.

This change is limited to test/build configuration (dev-deps) and does not alter runtime behavior for downstream users. It ensures the test-suite accurately reflects the capabilities exercised by the tests and prevents false negatives during CI.

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What changes are included in this PR?

• Modify datafusion/substrait/Cargo.toml dev-dependency for datafusion to add the unicode_expressions feature alongside nested_expressions.

Before:

[dev-dependencies]
datafusion = { workspace = true, features = ["nested_expressions"] }

After:

[dev-dependencies]
datafusion = { workspace = true, features = ["nested_expressions", "unicode_expressions"] }

No source code changes are required; this is a test/build configuration change.

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Are these changes tested?

• Yes — the change is intended to fix existing unit/integration tests that exercise SUBSTR/substring planning under the Substrait converter. In particular, the simple_scalar_function_substr test which failed with the NotImplemented error should now run with the UnicodeFunctionPlanner registered and pass.

• CI: this change is expected to make CI green for the Substrait test suite where the previous configuration caused a failure. Please run the repository test matrix locally or rely on CI to validate across all relevant feature combinations.

If maintainers prefer not to enable unicode_expressions in dev-deps, an alternative is to mark the test as conditionally skipped when unicode_expressions is not enabled or to add a fallback planning path for ASCII-only substring expressions. Both are noted below.

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Are there any user-facing changes?

• No. This change only affects development/test configuration and does not change public APIs or runtime behavior for consumers who do not enable unicode_expressions explicitly.

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Test plan / How to validate

1. Apply the patch (update datafusion/substrait/Cargo.toml).
2. Run the Substrait tests, or run cargo test -p datafusion-substrait --test substrait_integration
3. Confirm the simple_scalar_function_substr test passes and that no other tests regress.
4. Observe CI (GitHub Actions) to ensure the change resolves the previously failing job(s).

Context

This issue was uncovered here

[kosiew]

Thanks @adriangb for the review.