Asserts compute_save_path_display() never returns a backslash when the
save_dir is under the user's home directory, regardless of host OS.
Reproduces the original bug on Windows (failed message before the fix:
AssertionError: '\' unexpectedly found in
'~/l30d_save_path__luu2g76\Documents\Last30Days\british-airways-middle-east-raw-v3.md'
)
and locks in the contract on POSIX hosts (passes trivially today; would
fail if anyone removes .as_posix() in the future).
Verified by temporarily reverting the fix and confirming RED, then
re-applying the fix and confirming GREEN. All 13 CliV3Tests pass.
The Gemini 3.1 Flash Lite preview model is being discontinued on
May 25, 2026. Per Google's GA announcement, the underlying model
architecture is identical and only the model identifier needs to
be updated from `gemini-3.1-flash-lite-preview` to
`gemini-3.1-flash-lite`.
Also relaxes the `_require_gemini_31_preview` guard to accept any
`gemini-3.1-*` identifier (renamed to `_require_gemini_31`), so the
GA name and the still-preview `gemini-3.1-pro-preview` both pass.
Trevin Chow