feat(runfiles): implement runfiles.Path io methods

[rickeylev]

The runfiles.Path class is currently a PurePath subclass, which means
it only does string manipulation for constructing paths. This change
adds functionality to actually access files.

Fixes #3296

[gemini-code-assist]

Code Review

This pull request refactors the runfiles.Path class to inherit from pathlib.Path instead of pathlib.PurePath, enabling full I/O support such as stat, exists, and file reading, and adds comprehensive tests for these features. The review feedback identifies that the "Self" type hint is used in multiple method signatures without being imported or defined, which could cause type-checking failures; it is recommended to use "Path" for these forward references to ensure compatibility across Python versions.

[gemini-code-assist]

The type hint "Self" is used here and in several other methods throughout the class, but Self is not imported from typing (available in Python 3.11+) nor is it a defined class. This will cause issues with type checkers. Since this library maintains compatibility with older Python versions, you should use "Path" instead of "Self" for forward references to the current class.

Suggested change

	) -> "Self":
	) -> "Path":

[aignas]

The Self type annotation was only introduced in 3.13, this should be made compatible with lower Python versions too. :)