Use fs.pipe() for S3/GCS checkpoint uploads in _atomic_save (#21595)

* Use fs.pipe() for S3/GCS checkpoint uploads in _atomic_save

Replace fs.open() + f.write() with fs.pipe() for S3 and GCS
filesystems, enabling parallel multipart uploads. This gives
4-5x throughput improvement for checkpoints >= 500 MB.

Azure is excluded because adlfs stages blocks sequentially,
making fs.pipe() slower than f.write().

Fixes #21499

Signed-off-by: c-pozzi <corina.rios@gmail.com>

* Use isinstance check for Azure exclusion in _atomic_save

Use module_available + isinstance instead of string comparison
to detect AzureBlobFileSystem. Fix test mocking to patch
module_available so the isinstance check works without adlfs
installed.

Signed-off-by: c-pozzi <corina.rios@gmail.com>

* Fix ruff SIM117: combine nested with statements in tests

Signed-off-by: c-pozzi <corina.rios@gmail.com>

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Signed-off-by: c-pozzi <corina.rios@gmail.com>