Fixes#695
`where claude` on Windows returns both the npm-generated POSIX shell
wrapper (extensionless) and the Windows batch shim (`claude.cmd`)
side-by-side. The existing `path_from_successful_output` helpers in
`claude_cli.rs` and `codex_cli.rs` picked the first existing line,
which gave the POSIX wrapper - Windows tried to load it as a PE32,
`CreateProcessW` returned ERROR_BAD_EXE_FORMAT (193), and the AI chat
panel surfaced "%1 is not a valid Win32 application."
Mirror the fix that landed for Gemini in `gemini_discovery.rs`: when
`cfg!(windows)`, filter candidates through the existing shared utility
`cli_agent_runtime::has_windows_cli_extension` so .bat/.cmd/.com/.exe
win over extensionless POSIX scripts. Non-Windows behavior is unchanged.
Added a regression test in each file matching the
`windows_path_lookup_prefers_cmd_shim_over_extensionless_npm_script`
pattern from gemini_discovery.rs.
