* fix(providers): preserve openrouter/ prefix for native models (#12924)
OpenRouter-native models like 'openrouter/aurora-alpha' need the full
'openrouter/<name>' as the model ID in API requests. The existing
parseModelRef() stripped the prefix, sending just 'aurora-alpha'
which OpenRouter rejects with 400.
Fix: normalizeProviderModelId() now re-adds the 'openrouter/' prefix
for models without a slash (native models), while passing through
external provider models (e.g. 'anthropic/claude-sonnet-4-5') as-is.
Closes#12924
* Changelog: add OpenRouter note for #12942
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Co-authored-by: Luna AI <luna@coredirection.ai>
Co-authored-by: Vincent Koc <vincentkoc@ieee.org>
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