The forward-compat resolver hardcoded 'google' as the provider ID for
template lookup, so alias providers (google-vertex, google-gemini-cli)
could not find matching templates. Pass the actual provider ID from the
runtime context and add a templateProviderId fallback for cross-provider
template resolution.
Also fix flash-lite prefix ordering — check 'gemini-3.1-flash-lite'
before 'gemini-3.1-flash' to prevent misclassification.
Add regression tests for pro, flash, and flash-lite across provider
aliases.
Fixes#36111