Update benchmarks to remove custom env for prefetcher

[ankitaluthra1]

• Prefetcher is by default ON when cache_type is not SET, custom settings to use prefetcher with none is not needed anymore.

[gemini-code-assist]

Code Review

This pull request removes experimental read configurations from the benchmark Cloud Build pipeline and removes the explicit cache_type="none" parameter from several read operations in the performance microbenchmarks. The reviewer feedback highlights that removing cache_type="none" from random, mixed, and reopen read operations will cause them to default to readahead caching. This default behavior will trigger inefficient background prefetching and unnecessary GCS requests, which will introduce significant overhead and skew the benchmark results. It is recommended to retain cache_type="none" for these non-sequential read patterns.

[gemini-code-assist]

Removing cache_type="none" from random reads (_read_op_rand) will cause them to use the default cache_type="readahead". For random read patterns, readahead caching is highly inefficient and will trigger unnecessary GCS requests, severely degrading benchmark performance and skewing the results. We should keep cache_type="none" for random reads.

Suggested change

	open_kwargs = {"block_size": block_size}
	open_kwargs = {"block_size": block_size, "cache_type": "none"}

[ankitaluthra1]

Incorrect, the new default is none

[gemini-code-assist]

Removing cache_type="none" from reopen reads (_read_op_reopen) will cause them to use the default cache_type="readahead". For reopen benchmarks (which repeatedly open a file, read a single chunk, and close it), readahead caching will trigger background prefetching that is immediately wasted when the file is closed, adding significant overhead and skewing the connection overhead measurements. We should keep cache_type="none" for reopen reads.

Suggested change

	open_kwargs = {"block_size": block_size}
	open_kwargs = {"block_size": block_size, "cache_type": "none"}

[ankitaluthra1]

Incorrect, new default is none

[gemini-code-assist]

Removing cache_type="none" from mixed reads (_read_op_mixed) will cause them to use the default cache_type="readahead". Since mixed reads include random seeks, readahead caching can introduce significant overhead and skew the benchmark results. It is better to keep cache_type="none" for mixed reads.

Suggested change

	with gcs.open(path, "rb") as f:
	with gcs.open(path, "rb", cache_type="none") as f:

[ankitaluthra1]

Incorrect, new default is none

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.03%. Comparing base (8880f63) to head (59f8524).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #996   +/-   ##
=======================================
  Coverage   90.03%   90.03%           
=======================================
  Files          16       16           
  Lines        3593     3593           
=======================================
  Hits         3235     3235           
  Misses        358      358           

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