Support launching gemini.ps1 on Windows

Change gemini_helpers.get_gemini_executable to get_gemini_command and
teach it to search for gemini.ps1 in addition to .exe and friends. If
gemini.ps1 is found in PATH, return a command that will launch it via
PowerShell.

This allows commands such as:
  "vpython3 agents/extensions/install.py list"
to work on Windows when Gemini CLI is run via a .ps1 script in the
user's PATH.

Bug: 469100192
Change-Id: I0220ec71f86295519a8fc499e26e36cdfeb60eed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7633743
Reviewed-by: Erik Staab <estaab@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Auto-Submit: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1595675}
NOKEYCHECK=True
GitOrigin-RevId: 0c765f74922a5930069a18b154dd484c217c4065