Adjust ECB data expectations in 2 tests

[khaeru]

Address 3 tests that were failing on CI.

• Similar to Adjust expectation for test_request_preview_data() #233, match current data returned by ECB data source for:
  • TestClient.test_preview_data()
  • test_doc_usage_data()
• Fix Address XHTML validation error with lxml 6.0.0 #235:
  • Adjust .format.xml.common.construct_schema(), added in Miscellaneous improvements from 2025-W08 #227: fetch and use a local copy of xhtml1-strict.xsd, instead of referencing by its URL.

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 98.10%. Comparing base (f042e75) to head (3aeb913).
⚠️ Report is 87 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #236      +/-   ##
==========================================
- Coverage   98.92%   98.10%   -0.83%     
==========================================
  Files         103      103              
  Lines        8683     8687       +4     
==========================================
- Hits         8590     8522      -68     
- Misses         93      165      +72     

Files with missing lines	Coverage Δ
sdmx/format/xml/common.py	100.00% <100.00%> (ø)
sdmx/testing/__init__.py	99.23% <100.00%> (+<0.01%)	⬆️
sdmx/tests/test_client.py	100.00% <100.00%> (ø)
sdmx/tests/test_docs.py	100.00% <100.00%> (ø)
sdmx/tests/writer/test_writer_xml.py	100.00% <100.00%> (ø)

... and 20 files with indirect coverage changes

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