feat(api): add NanoGPT provider support (#1239)

* feat: add NanoGPT provider backend

* feat: add NanoGPT provider settings

* fix: harden NanoGPT provider boundaries

* no-mistakes(review): Handle NanoGPT null metadata and reasoning capabilities

* no-mistakes(review): Fix NanoGPT cache-capable request routing

* fix(api): harden NanoGPT metadata handling

* chore: retrigger checks

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Co-authored-by: @taltas <6816042+taltas@users.noreply.github.com>