Adjust expectation for test_request_preview_data()

[khaeru]

Around 2025-06-09, data returned by the ECB data source changed, causing one test to fail. Compare A versus B. Specifically, series keys returned by the query https://data-api.ecb.europa.eu/service/data/EXR?detail=serieskeysonly changed, such that the number of keys matching a particular filter increased from 24 to 30:

        # A filter can be provided, resulting in fewer keys
        keys = ECB.preview_data("EXR", {"CURRENCY": "CAD+CHF+CNY"})
>       assert len(keys) == 24
E       assert 30 == 24
E        +  where 30 = len([<SeriesKey: FREQ=A, CURRENCY=CAD, CURRENCY_DENOM=EUR, EXR_TYPE=SP00, EXR_SUFFIX=A>, <SeriesKey: FREQ=Q, CURRENCY=CAD, CURRENCY_DENOM=EUR, EXR_TYPE=SP00, EXR_SUFFIX=A>, <SeriesKey: FREQ=H, CURRENCY=CAD, CURRENCY_DENOM=EUR, EXR_TYPE=SP00, EXR_SUFFIX=A>, <SeriesKey: FREQ=M, CURRENCY=CAD, CURRENCY_DENOM=EUR, EXR_TYPE=SP00, EXR_SUFFIX=A>, <SeriesKey: FREQ=D, CURRENCY=CAD, CURRENCY_DENOM=EUR, EXR_TYPE=SP00, EXR_SUFFIX=A>, <SeriesKey: FREQ=Q, CURRENCY=CAD, CURRENCY_DENOM=EUR, EXR_TYPE=BRC0, EXR_SUFFIX=A>, ...])

This PR adjusts.

Also:

• Organize and type hint tests in the one module.
• Dead link for sdmx information model in readme #232.

PR checklist

• Checks all ✅
• Update documentation.
• Update doc/whatsnew.rst

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.09%. Comparing base (abb42b5) to head (d3cb374).
⚠️ Report is 41 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #233      +/-   ##
==========================================
- Coverage   98.92%   98.09%   -0.83%     
==========================================
  Files         103      103              
  Lines        8669     8683      +14     
==========================================
- Hits         8576     8518      -58     
- Misses         93      165      +72     

Files with missing lines	Coverage Δ
sdmx/client.py	96.17% <100.00%> (-2.74%)	⬇️
sdmx/model/v30.py	100.00% <100.00%> (ø)
sdmx/testing/__init__.py	99.22% <100.00%> (+<0.01%)	⬆️
sdmx/testing/data.py	100.00% <100.00%> (ø)
sdmx/tests/model/test_v21.py	100.00% <100.00%> (ø)
sdmx/tests/test_client.py	100.00% <100.00%> (ø)
sdmx/tests/test_dataset.py	100.00% <100.00%> (ø)
sdmx/tests/test_docs.py	100.00% <100.00%> (ø)
sdmx/util/requests.py	100.00% <100.00%> (ø)

... and 19 files with indirect coverage changes

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