feat: expose interpreter files-to-run on PyRuntimeInfo

[jsun-splunk]

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 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.

[gemini-code-assist]

Code Review

This pull request introduces the interpreter_files_to_run field to PyRuntimeInfo, allowing action consumers to execute an in-build runtime interpreter along with its runfiles. The changes include updates to the py_runtime rule to populate this field when an executable target is provided, validation logic to ensure consistency between the interpreter and its runfiles provider, and comprehensive tests to verify the new functionality and error handling. I have no feedback to provide.