fix(streaming): return empty bytes when reading at the end of an S3 object

[chancesong10]

Issue number: closes #8510

Summary

Changes

_S3SeekableIO opens each stream with Range: bytes=<position>-, and S3 answers 416 InvalidRange when that position is at or past the end of the object, which includes any range on an empty object.

• raw_stream catches ClientError with code InvalidRange and uses an empty PowertoolsStreamingBody, so reads at the end return b"" like a regular file. Any other error is still raised. This keeps one GetObject per stream, with no extra HeadObject to check the size first.
• close() only closes _raw_stream if a stream is already open. Before, it went through the raw_stream property, so closing an unread object made a GetObject just to close it, and closing at the end of the object raised.

PowertoolsStreamingBody is now imported at runtime rather than only under TYPE_CHECKING, since it's used to build the empty body.

Tests added to tests/functional/streaming/_boto3/test_s3_seekable_io.py:

• test_read_at_end_of_object_returns_empty_bytes
• test_read_empty_object_returns_empty_bytes, covering read() and iteration
• test_raw_stream_raises_other_client_errors (NoSuchKey still raises)
• test_context_manager_at_end_of_object

test_closed no longer stubs a get_object. It now checks that closing an unread object makes no request. The three new behaviour tests and the updated test_closed fail on develop.

User experience

Before:

obj = S3Object(bucket="my-bucket", key="data.txt")
obj.seek(0, io.SEEK_END)
obj.read()        # ClientError: InvalidRange

S3Object(bucket="my-bucket", key="photos/").read()  # empty object: ClientError: InvalidRange

with S3Object(bucket="my-bucket", key="data.txt") as obj:
    obj.seek(0, io.SEEK_END)
# ClientError: InvalidRange, raised by close() on exit

After, these return b"", b"", and exit cleanly, the same as io.BytesIO. Closing an unread object no longer calls GetObject.

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[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.92%. Comparing base (b1af7e8) to head (164497c).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8511      +/-   ##
===========================================
+ Coverage    96.88%   96.92%   +0.04%     
===========================================
  Files          317      317              
  Lines        15899    15910      +11     
  Branches      1395     1398       +3     
===========================================
+ Hits         15403    15421      +18     
+ Misses         361      353       -8     
- Partials       135      136       +1     

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