Avoid underflow in default ranges before a BOM

[ntBre]

Summary

This fixes a regression caused by the BOM handling in #19806. Most diagnostics already account for the BOM in their ranges, but those that use TextRange::default to mean the beginning of the file do not, causing an underflow in RenderableAnnotation::new when subtracting the BOM-shifted snippet_start from the annotation range.

I ran into this when trying to run benchmarks on CPython in preparation for caching work. The file cpython/Lib/test/bad_coding2.py was causing a crash because it had a default-range I002 diagnostic, with a BOM.

ruff/crates/ruff_linter/src/rules/isort/rules/add_required_imports.rs

Lines 122 to 126 in 7cc3f1e

	// Always insert the diagnostic at top-of-file.
	let mut diagnostic = context.report_diagnostic(
	MissingRequiredImport(required_import.to_string()),
	TextRange::default(),
	);

The fix here is just to saturate to zero instead of panicking. I considered adding a TextRange::saturating_sub method, but I wasn't sure it was worth it for this one use. I'm happy to do that if preferred, though.

Saturating seemed easier than shifting the affected annotations over, but that could be another solution.

Test Plan

A new ruff_db test that reproduced the issue and manual testing against the CPython file mentioned above

[MichaReiser]

Is this marked as internal because the change hasn't been released yet?

[ntBre]

Is this marked as internal because the change hasn't been released yet?

Yep, that was my thinking. Otherwise it's a bug.