test(install): cover skipping submodule init when none declared (#37702)

[xxxigm]

What does this PR do?

Adds test-only coverage for the Windows-installer fix proposed in #37702, which makes scripts/install.ps1 skip git submodule update --init --recursive when the checkout declares no submodules.

This PR does not modify scripts/install.ps1 — it only adds tests. The suite skips itself until the #37702 guard is present, so it stays green on main today and activates automatically once #37702 lands (or is applied alongside).

Background

Hermes ships no .gitmodules, yet the installer always ran git submodule, which on some Windows hosts fails through Git-for-Windows' shell-helper path (git-sh-i18n: No such file or directory). #37702 wraps the step in if (Test-Path ".gitmodules") and prints a skip notice otherwise. That PR left the test checklist items unticked; this PR supplies them.

Type of Change

• Tests (adding or improving test coverage)

Changes Made

• tests/test_install_ps1_skip_empty_submodules.py (new):
  • Static guards (portable, run everywhere):
    • git submodule update --init --recursive lives inside the if (Test-Path ".gitmodules") branch.
    • the else branch prints No submodules declared; skipping submodule init.
    • exactly one (guarded) submodule-update call exists — no unconditional duplicate.
  • Behavioral tests (run under PowerShell, skipped when pwsh/powershell is absent, e.g. Linux/macOS CI): execute the extracted block with a stubbed git and assert git submodule runs only when .gitmodules is present.
  • The whole suite skips with a clear reason until install.ps1 contains the fix(install): skip submodule init when none declared #37702 guard, so it never fails on main before the fix merges.

How to Test

pytest tests/test_install_ps1_skip_empty_submodules.py -v

• On current main (no guard yet): 5 skipped.
• With the fix(install): skip submodule init when none declared #37702 guard applied: 3 passed, 2 skipped (the 2 behavioral tests run when PowerShell is installed).

Documentation

Reviewed docs for submodule references. The installer change has no user-facing docs to update. The remaining git submodule mentions (website/docs/getting-started/termux.md, updating.md) describe manual git workflows and are harmless no-ops on a repo without .gitmodules — out of scope here.

Checklist

• Conventional Commit messages
• pytest tests/test_install_ps1_skip_empty_submodules.py -q passes (5 skipped on main; 3 passed / 2 skipped with the fix)
• Tests added
• Documentation reviewed — N/A for installer internals

[ethernet8023]

won't fix. see #37702