* feat: add GLM-5.3 Flash to Z.AI model list
Add glm-5.3-flash next to glm-5.3 in _PROVIDER_MODELS["zai"] and mirror
it in _FALLBACK_MODELS, following the existing glm-4.5 / glm-4.5-flash
sibling pattern. Reasoning-effort gating needs no change: the >= 5.2
version gate already classifies the id at the full effort tier.
Adds a regression suite asserting catalog presence, label, ordering,
fallback entry, and end-to-end propagation through
get_available_models().
* test: pin glm-5.3-flash sibling adjacency in zai provider catalog
Per PR review feedback (#7323): the provider-catalog ordering test now
asserts glm-5.3-flash immediately after glm-5.3 and glm-5.2 immediately
after the flash sibling, matching the adjacency the fallback test
already pins. The base<flash relative check is subsumed; flash<glm-5.1
is kept to pin 5.1 after the whole 5.3 generation.
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Co-authored-by: nesquena-hermes <nesquena+hermes@gmail.com>
