Fix AIS subprocess lifetime when opening datastore objects

[fallintoplace]

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What does this PR do ?

Fix open_datastore_object_with_binary() so AIS-backed file objects stay usable after the helper returns.

Collection: Common

Changelog

• replace the with subprocess.Popen(...) return path with a small stream wrapper that owns the AIS subprocess
• keep the subprocess alive until the caller closes the returned stream, then close the pipes and wait for the process
• add unit coverage for the success path and the retry/error path around AIS binary reads

Usage

with open_datastore_object_with_binary("ais://bucket/object") as stream:
    chunk = stream.read(4096)

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Additional Information

• Related to #
• I couldn't run pytest in this checkout because the active interpreter is missing the project test dependencies, but I did add focused unit coverage and exercised the changed subprocess path with a direct smoke check.

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