Preserve separator in core path ops

[p1c2u]

This pull request improves the handling of custom path separators in the BasePath class and ensures that all relevant methods and operations respect the separator specified by the user. It also adds comprehensive tests to verify this behavior.

Enhancements to separator handling in BasePath:

• Updated with_name to check for the instance's separator instead of a global constant, ensuring correct validation for custom separators.
• Modified is_relative_to and relative_to to pass the instance's separator to parse_args, guaranteeing correct parsing when custom separators are used. [1] [2]
• Changed __rtruediv__ to preserve the separator when constructing new paths, so that operations like "a" / BasePath("b.c", separator=".") maintain the correct separator.

Expanded test coverage for custom separators:

• Added test_preserves_separator to verify that the separator is preserved in division operations.
• Added test_with_name_respects_separator to ensure that with_name raises an error when the new name contains the custom separator.
• Added test_relative_to_and_is_relative_to_custom_separator to check that relative path operations work correctly with custom separators.

[Copilot]

Pull request overview

This PR ensures BasePath operations consistently respect an instance’s custom separator, fixing cases where methods accidentally used the default global separator and adding tests to prevent regressions.

Changes:

• Update with_name validation to reject names containing self.separator (not the global default).
• Pass sep=self.separator into parse_args for is_relative_to and relative_to.
• Preserve separator in __rtruediv__ path construction (e.g., "a" / BasePath(..., separator=".")).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
pathable/paths.py	Propagates instance separator through name validation, relative path parsing, and reverse-division construction.
tests/unit/test_paths.py	Adds targeted tests covering separator preservation and relative/name ops with custom separators.

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