TEST: Cover absolute-size margin warning

[initial-d]

Adds a regression test for absolute-sized orders that are canceled because there is not enough margin.

This covers the behavior discussed in #1360. The implementation on current 'master' already emits a UserWarning for the absolute-size path; this test makes that behavior explicit and keeps it aligned with the existing relative-size warning path.

Validation:

• 'python -m py_compile backtesting/test/_test.py' passes
• 'git diff --check' passes
• Tried to run the targeted unittest locally, but the available Python environment was missing test/runtime dependencies such as 'bokeh'. I started a local .venv install, but dependency installation took too long in this environment, so I left full execution to CI.

Fixes #1360

[kernc]

Ah, thanks for checking (and proving) that!

[initial-d]

Thanks for merging it, and thanks for the quick review.