Fix mock content initialization in test fixtures

[metaodi]

This commit addresses test failures caused by mock objects being passed to XML parsing functions. The issue occurred when test fixture files couldn't be found, leaving response_mock.content uninitialized as a Mock object instead of bytes/string.

Changes:

1. Initialize response_mock.content to empty string when no fixture file is found, preventing TypeError in XML parsing
2. Add fallback logic to handle fixture file naming mismatches between snake_case test method names and CamelCase fixture files
3. Implement case-insensitive fixture file lookup for backward compatibility
4. Fix black formatting issue in osmapi/OsmApi.py (add blank line after module docstring)
5. Update test_Capabilities_deprecation_warning to explicitly specify fixture file

The fixture lookup now tries multiple strategies:

• Exact filename match
• Case-insensitive match
• snake_case to CamelCase conversion (e.g., test_way_get.xml -> test_WayGet.xml)

This fixes the test_Capabilities_deprecation_warning test and many other tests that were failing due to fixture file naming mismatches after the snake_case refactoring.

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