chore: Enable unused_async lint

[neilconway]

Which issue does this PR close?

• Closes: N/A

Rationale for this change

There are a reasonable number of places where we have functions marked as async that don't need to be so. Fix this by enabling the unused_async lint, and fixing up the resulting breakage. There are a handful of spots that need an expect(clippy::unused_async) -- mostly mocks of async methods and example code.

What changes are included in this PR?

• Enable unused_async lint
• Remove unnecessary async annotations in a bunch of places
• Add expect(clippy::unused_async) where necessary

Are these changes tested?

Yes, covered by existing tests (no behavioral change expected).

Are there any user-facing changes?

Yes: this PR updates a few public APIs:

• datafusion_cli::command::OutputFormat::execute
• datafusion::test_util::parquet::TestParquetFile::create_scan
• datafusion_datasource_csv::file_format::CsvFormat::read_to_delimited_chunks_from_stream
• datafusion_substrait::serializer::deserialize_bytes

Migration is mostly straightforward (e.g., removing await from calling code).

[neilconway]

If it is preferred, I can revert the changes to the public APIs and add expect to those places instead.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 68.75000% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.67%. Comparing base (1e58928) to head (52344c9).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/physical-plan/src/limit.rs	22.72%	0 Missing and 17 partials ⚠️
benchmarks/src/imdb/run.rs	0.00%	2 Missing ⚠️
datafusion/datasource-parquet/src/metadata.rs	33.33%	0 Missing and 2 partials ⚠️
.../src/logical_plan/consumer/expr/scalar_function.rs	89.47%	0 Missing and 2 partials ⚠️
datafusion/physical-plan/src/sorts/sort.rs	80.00%	0 Missing and 1 partial ⚠️
datafusion/substrait/src/serializer.rs	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23679      +/-   ##
==========================================
- Coverage   80.67%   80.67%   -0.01%     
==========================================
  Files        1086     1087       +1     
  Lines      366800   367527     +727     
  Branches   366800   367527     +727     
==========================================
+ Hits       295912   296491     +579     
- Misses      53260    53374     +114     
- Partials    17628    17662      +34     

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