test(ai-conversations): add 36 ERT tests covering helpers and entry points

ai-conversations.el shipped without direct tests.  This file covers
every helper and interactive entry point across Normal / Boundary /
Error.

Helpers: `cj/gptel--slugify-topic` (ASCII, empty input, all-special,
unicode stripped, idempotent, trim, digits); `cj/gptel--timestamp-
from-filename` (normal decode, year-edge boundaries, malformed
inputs returning nil); `cj/gptel--existing-topics` and `cj/gptel--
latest-file-for-topic` (multi-topic / multi-timestamp temp dirs,
empty dir, missing dir, prefix-overlap isolation); `cj/gptel--
conversation-candidates` (newest-first and oldest-first sort order,
display-string shape, error on missing dir); `cj/gptel--save-buffer-
to-file` (visibility headers prepended, round-trip through `cj/
gptel--strip-visibility-headers`).

Autosave: post-response hook saves only when gptel-mode + enabled +
filepath are all set; autosave-after-send swallows write errors via
`message` instead of signaling; the install-once guard prevents
double-registration.

Interactive entry points: save/delete exercised via `cl-letf` stubs
on `completing-read` and `y-or-n-p`.

Per-test temp directories; no writes outside them.

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