Migrate T-digest test helpers to memory_resources

[bdice]

Description

Update the installed T-digest generators, expected-column builders, validators, and shared groupby/reduction drivers to accept cudf::memory_resources. Returned T-digest columns use the output resource; input construction, intermediate results, and validation scratch use the temporary resource.

This allows T-digest tests to control their setup and validation allocations without falling back to the current device resource.

This is a non-breaking change for existing callers of these test helpers.

Depends on #23581.

Part of #20780

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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