test: add unit tests for PR changes

[coderabbitai]

Unit test generation was requested by @RussellSB.

• feat: add weekly data support and rework date logic #205 (comment)

The following files were modified:

• tests/test_conftest_helpers.py
• tests/test_index_dtype_resolution.py
• tests/test_io_results.py
• tests/test_non_dates.py
• tests/test_process_signals.py
• tests/tests_plotting/edgecases/test_plot_pytrendy_index_types.py

[coderabbitai]

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