review: seed ReadinessCache with dict.fromkeys (Sonar S7519); changelog

S8714 (test try/except in tests/test_live_capabilities.py) accepted in SonarCloud
with rationale: it is a unit-test rule applied to an operator-facing live gate
that consumes an arbitrary base URL, where the caught body preview is the
diagnostic that reveals an operator dialing the wrong endpoint (200 + HTML).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MrghbUdRuVa2Y98EV8RHQg

Author: OriNachum

CHANGELOG.md

@@ -35,6 +35,7 @@ adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 - **#92 / #95** — the gateway never advertises an origin built from its internal listen port. Precedence is `GATEWAY_PUBLIC_URL` (an operator override for a tunnel or Host-rewriting proxy) > the request `Host` header > an empty endpoint. `GATEWAY_PUBLIC_URL` is deliberately NOT defaulted: a defaulted `public_url` outranks `Host` and would advertise loopback to every remote client.
 - **#96** — `AUDIO_URL` now reaches the gateway from the base fleet compose, so `stt`/`tts` stop advertising `ready=true` on a path that 404s when the audio overlay is not composed in.
 - **#97** — `GET /v1/models` no longer advertises phantom backends wired from a `*_SERVED_NAME` alone against a `default_url` naming a container that need not exist.
+- `ReadinessCache` seeds its initial per-backend snapshot with `dict.fromkeys` instead of a dict comprehension (SonarCloud `python:S7519`).
 - `ReadinessCache.stop()` no longer clears its thread reference while the refresh thread is still alive, so a subsequent `start()` cannot spawn a second concurrent refresh loop; the join bound now covers a full sequential refresh pass and `start()` cleanly replaces a genuinely-dead thread (Qodo reliability).
 - An unknown model id returns `404 model_not_found` instead of being silently served by the default backend under a different model's weights. Unknown-ness is decided against the routing table, never against the readiness-filtered `/v1/models` list, so a wired-but-dead backend still yields 503 rather than 404.
 - `test_live_main_text_returns_nonempty_content` no longer fails on a thinking model: `max_tokens=16` was consumed entirely by the reasoning trace, leaving `content=None` and `finish_reason=length` (issue #93).