feat: expose interpreter files-to-run on PyRuntimeInfo (#3795)

Rules that execute a py_runtime interpreter in an action need the
interpreter executable together with its runfiles metadata. The existing
PyRuntimeInfo fields identify the interpreter file and runtime files,
but do not preserve the target's FilesToRunProvider for executable
interpreter targets.

Add PyRuntimeInfo.interpreter_files_to_run for runtimes created from an
executable interpreter target, and validate that direct provider
construction keeps the FilesToRunProvider executable aligned with the
interpreter field. Direct file interpreters and platform runtimes
continue to leave this field unset, preserving existing py_runtime
behavior.

Document the new public provider field and add focused analysis-test
coverage for executable, file-only, platform, and invalid constructor
cases.

One example is
[rules_pycross](https://github.com/jvolkman/rules_pycross) wheel
building with an in-build executable Python runtime. pycross needs to
run the selected Python interpreter in an action, but PyRuntimeInfo
previously exposed only the interpreter File and runtime files, not the
interpreter target's FilesToRunProvider. For executable in-build
runtimes, the FilesToRunProvider is the Bazel-native handle that carries
both the executable and the runfiles metadata needed to stage it as an
action tool. Exposing it lets pycross consume the runtime interpreter
directly for wheel-build actions instead of reconstructing or
approximating the interpreter's runtime closure from separate fields.

Co-authored-by: Richard Levasseur <richardlev@gmail.com>