fix: raise when presigned URL response is missing or malformed

[Ma77Ball]

What changes were proposed in this PR?

DatasetFileDocument.get_presigned_url previously returned response.json().get("presignedUrl") on a 200 response, so a body that omitted the key (or wasn't valid JSON) silently returned None. This PR parses the JSON inside a try/except block and validates that presignedUrl is a non-empty string; otherwise, it raises RuntimeError with the response body.

Any related issues, documentation, or discussions?

Closes: #4725

How was this PR tested?

Manually reproduced the missing-key case (mocked 200 + empty JSON) and confirmed the new RuntimeError is raised at the presign step instead of a downstream requests.get(None) failure.

Was this PR authored or co-authored using generative AI tooling?

Co-authored with Claude Opus 4.7 in compliance with ASF

[Ma77Ball]

/request-review @aicam

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.02%. Comparing base (13c4836) to head (0c34245).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5073      +/-   ##
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- Coverage     43.15%   43.02%   -0.13%     
+ Complexity     2213     2201      -12     
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  Files          1045     1045              
  Lines         40353    40147     -206     
  Branches       4267     4229      -38     
============================================
- Hits          17413    17272     -141     
+ Misses        21865    21815      -50     
+ Partials       1075     1060      -15     

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[aicam]

LGTM. Replaces a silent None return (which previously caused requests.get(None) downstream in read_file() with a confusing TypeError) with explicit RuntimeErrors for the three failure modes: invalid JSON, missing key, and non-string/empty value. Error messages quote response.text for diagnosability.

Verified locally (Python 3.12)

$ pytest src/test/python/pytexera/storage/test_dataset_file_document.py -v
============================== 21 passed in 0.50s ==============================

All 21 tests pass, including the 3 new negative-path tests (test_raises_when_response_body_lacks_presigned_url_key, test_raises_when_response_body_is_not_valid_json, test_raises_when_presigned_url_is_empty_string, test_raises_when_presigned_url_is_not_a_string).

Tight, focused fix.