fix(bootstrap): restore imports for native Python targets

[gleyba]

Fixes #4234. Reproductions and failure details are in
the issue.

Bazel 7's built-in Python rules leave newer launcher placeholders unexpanded
when using a rules_python runtime. The Python bootstrap crashes while parsing
%runtime_venv_symlinks%; the shell bootstrap passes %interpreter_args% to
Python as a script filename. After bypassing the Python startup crash, its
direct-to-main fallback also omits declared import roots.

This completes the legacy compatibility path discussed in
PR #3495, without
changing normal stage-2 launches or the project's support policy.

Changes

• Treat the unexpanded venv-symlink placeholder as an empty mapping.
• Add the native main-file fallback to the shell launcher and clear its
  unexpanded interpreter-argument token only in that fallback.
• Restore native import ordering in both fallbacks using declared imports and
  Bazel's workspace/repository settings; preserve inherited PYTHONPATH.
• Strip the native workspace/../ prefix from shell entry-point paths so an
  external binary does not depend on the workspace directory existing.
• Add shared template-level regression tests for both launchers and a news
  entry for the generated changelog.

After the change, native hello targets start successfully and declared
dependencies are importable. Existing stage-2 import behavior and interpreter
arguments remain unchanged. No public APIs or build flags are added or changed.

Minimal reproduction

For the unpatched shell-launcher failure, use these four files:

# .bazelversion
7.7.1

# MODULE.bazel
bazel_dep(name = "rules_python", version = "2.4.1")

# BUILD.bazel — intentionally use the built-in rule, without a load()
py_binary(name = "hello", srcs = ["hello.py"])

# hello.py
print("launcher reached main")

bazelisk run --@rules_python//python/config_settings:bootstrap_impl=script //:hello

Before: Python tries to open the literal %interpreter_args% filename.
After: the program prints launcher reached main.

The issue also contains the unpatched Python-launcher reproduction and an
explicitly crash-guarded example exposing the subsequent import failure.
These use Bzlmod and default runtimes, without custom toolchains or runfiles
manipulation.

Validation

All six targets passed locally with Bazel 9.1.1:

bazel test --config=fast-tests \
  //tests/bootstrap_impls:native_bootstrap_test \
  //tests/bootstrap_impls:native_bootstrap_script_test \
  //tests/bootstrap_impls:interpreter_args_test \
  //tests/bootstrap_impls:run_binary_bootstrap_script_zip_no_test \
  //tests/bootstrap_impls:run_binary_bootstrap_script_find_runfiles_test \
  //tests/bootstrap_impls:stdlib_shadowing_system_python_test

The new tests render the templates with native Bazel 7 substitutions; they do
not enable deprecated native rules in Bazel 9. They cover startup, import
ordering, workspace and external repository imports, inherited PYTHONPATH,
and unchanged stage-2 interpreter arguments.

The Bzlmod reproductions also pass against this fix with Bazel 7.7.1, including
the shell launcher, an external native target, and declared imports.
Buildifier, Ruff, and Bash syntax checks passed.

The release note is in news/native-bootstrap-imports.fixed.md, following the
contribution guideline to use news fragments instead of editing the generated
CHANGELOG.md directly.