`E501`/`W505`/formatter: Exclude nested pragma comments from line width calculation (#24071)
Closes #18470
## Summary
Exclude nested pragma comments like the `# noqa` in `test # comment 1 # noqa` from `E501`, `W505` and the formatter's line width calculation.
## Test Plan
- Added linter test fixture `E501_5.py` with preview test cases for
nested pragmas (`# comment #noqa`, `## noqa`, `# comment # type:
ignore`)
- Added formatter test fixture `trailing_pragma_nested.py` demonstrating
stable vs preview reserved width behavior
- Doctests for `find_trailing_pragma_offset` in `pragmas.rs`
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Co-authored-by: Micha Reiser <micha@reiser.io>