[physical-plan]: rename aggregate stream modules

[alamb]

Which issue does this PR close?

• Closes #.

Rationale for this change

Several aggregate stream implementations are stored in modules whose file names do not match the primary stream types they contain. This makes the aggregate module harder to scan because some stream files use the _stream.rs suffix while others are named after older implementation details.

What changes are included in this PR?

This PR renames aggregate stream modules so their filenames match the stream implementations more closely:

• no_grouping.rs -> aggregate_stream.rs
• hash_aggregate.rs -> hash_stream.rs
• row_hash.rs -> grouped_hash_stream.rs
• topk_stream.rs -> grouped_topk_stream.rs

It also updates private module declarations, imports, and doc links/comments that referenced the old module names.

Are these changes tested?

No new tests are included because this is a module/file rename with no behavioral changes.

Validated with:

• cargo fmt --all -- --check
• cargo check -p datafusion-physical-plan

Are there any user-facing changes?

No. These modules are private implementation details of datafusion-physical-plan.

[alamb]

Closing in favor of a clean rename-only draft PR from codex/rename-aggregate-stream-modules.