Remove GroupsAccumulator::supports_convert_to_state and require convert_to_state

[lyne7-sc]

Which issue does this PR close?

• Closes Remove GroupsAccumulator::supports_convert_to_state and make convert_to_state mandatory #23081.

Rationale for this change

Following #23275, all GroupsAccumulator implementations now provide convert_to_state.

The supports_convert_to_state capability flag is therefore no longer needed.

What changes are included in this PR?

• Make GroupsAccumulator::convert_to_state a required trait method.

• Remove GroupsAccumulator::supports_convert_to_state and its implementations.

• Remove the corresponding capability checks from hash aggregation.

• Simplify skip-partial aggregation to use the required convert_to_state implementation directly.

• Add a regression test covering the partial hash aggregation skip path.

• Document the breaking trait change in the 55.0.0 upgrading guide.

• Remove FFI_GroupsAccumulator::supports_convert_to_state. This changes the FFI ABI layout, so providers and consumers must be rebuilt against DataFusion 55.

Are these changes tested?

Yes. Added a regression test verifying that skip-partial aggregation uses the required convert_to_state implementation without a capability flag.

Existing physical-plan and FFI tests continue to pass.

Are there any user-facing changes?

Yes. This is a breaking Rust API change for external GroupsAccumulator implementations:

• convert_to_state must now be implemented.
• supports_convert_to_state should be removed.

The migration is documented in the 55.0.0 upgrading guide.

The FFI_GroupsAccumulator layout has changed. FFI providers and consumers must be rebuilt against DataFusion 55 and must not exchange this struct with older major versions.

[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
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--- failure trait_method_default_impl_removed: pub trait default method impl removed ---

Description:
A method's default impl in an unsealed trait has been removed, breaking trait implementations that relied on that default
        ref: https://doc.rust-lang.org/book/ch10-02-traits.html#default-implementations
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.48.0/src/lints/trait_method_default_impl_removed.ron

Failed in:
  trait method datafusion_expr_common::groups_accumulator::GroupsAccumulator::convert_to_state in file /home/runner/work/datafusion/datafusion/datafusion/expr-common/src/groups_accumulator.rs:232

--- failure trait_method_missing: pub trait method removed or renamed ---

Description:
A trait method is no longer callable, and may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#trait-item-signature
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.48.0/src/lints/trait_method_missing.ron

Failed in:
  method supports_convert_to_state of trait GroupsAccumulator, previously in file /home/runner/work/datafusion/datafusion/target/semver-checks/git-apache_main/da6043036f7c1e9a11fb1a08008a9adb854dcfe7/datafusion/expr-common/src/groups_accumulator.rs:242

     Summary semver requires new major version: 2 major and 0 minor checks failed
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