Remove `GroupsAccumulator::supports_convert_to_state` and require `convert_to_state` (#23489)

## Which issue does this PR close?

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- Closes #23081.

## Rationale for this change

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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?

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- 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?

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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?

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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.