[helly25]

Add tools = {"GENERATOR": "//tools:generator"} to build helper executables for the platform running a test and export their resolved paths automatically. Scripts invoke "${GENERATOR}"; helper runfiles are included, including Windows executables and manifest-only layouts.

AG;DR

• Targets main; dependency Support Windows Bash and manifest-only runfiles #54 has merged. Merge this PR before Provision Bash and Unix utilities automatically on Windows #56. The synchronization merge preserves the previously tested files exactly: Support Windows Bash and manifest-only runfiles #54 was squash-merged with an identical tree.
• A Starlark test wrapper transitions tools with config.exec("test"), preserving the test execution group's platform constraints. A non-test sh_binary target supplies the rules_shell launcher; a generated manifest maps environment names to executable runfiles. With tools there is exactly one test target, with no forced manual tag or private visibility on the launcher.
• Preserve target-configured data, helper default/data runfiles, inherited test environment, and coverage metadata. Multiple environment names can reference one executable. The test rule owns environment expansion, preserving runfiles-relative location paths on Bazel 7 and 9.
• Reject non-executable tools, malformed/reserved variable names, environment collisions, and targets declared directly in both data and tools. Avoid mixing target- and execution-configured instances of the same generated runfile in transitive dependencies.
• Tests exercise two distinct generated executables in one script, runtime dependencies, aliases, quoted arguments, renamed external repositories, paths containing spaces, manifest-only bazel test and bazel run, and invalid declarations. The external consumer selects different ordinary and test execution platforms on Bazel 9; Bazel 7 uses its supported exec_compatible_with attribute.
• tool_fixture.bzl is private test infrastructure, with private load visibility; it is not part of the public API.
• Validation of head 1ae692c: local bazel test //... (8 passing) and repository-wide pre-commit pass. All fresh PR CI checks against main passed, including Linux, macOS, Windows, release-site checks, pre-commit, and the required done gate. Post-merge main CI for Support Windows Bash and manifest-only runfiles #54 also passed.