Replace live gcs fetch test with mocked

[rfbgo]

Previously we were hitting a live end point out of our control (gs://hpc-toolkit-demos/build_cache/index.json), which means our unit tests required internet. This addresses those 2 issues

[gemini-code-assist]

Code Review

This pull request refactors the GCS fetch strategy tests by replacing the network-dependent test with a mocked implementation using custom mock classes (MockBlob, MockBucket, and MockGcsClient). Additionally, it introduces a new test, test_gcsfetchstrategy_content_type_mismatch, to ensure a warning is issued when there is a content type mismatch. There are no review comments, so I have no feedback to provide.

[ramble-pr-bot]

Ramble Performance Test Metrics

Results produced with commit: 580d1b3

Test Name	Outcome	Duration (s)	Most Recent Run (s)	Last 5 Avg (s)
test_analyze_large_file	passed	2.0646	2.0856 (b311f41)	2.0701
test_large_template_expansion	passed	1.1958	1.2188 (b311f41)	1.1978
test_many_experiments	passed	24.1248	25.3640 (b311f41)	24.4371
test_many_objects_defaults	passed	13.1091	13.6953 (b311f41)	12.7329
test_matrix_filter_perf	passed	1.1221	1.1669 (b311f41)	1.1093

[codecov]

Codecov Report

❌ Patch coverage is 96.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.09%. Comparing base (b311f41) to head (580d1b3).

Files with missing lines	Patch %	Lines
lib/ramble/ramble/test/gcs_fetch.py	96.42%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1753      +/-   ##
===========================================
+ Coverage    94.07%   94.09%   +0.01%     
===========================================
  Files          370      370              
  Lines        37424    37463      +39     
===========================================
+ Hits         35207    35251      +44     
+ Misses        2217     2212       -5     

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[rfbgo]

@dcoapp recheck

[rfbgo]

@dcoapp recheck