v0.2.1: failover now handles a bad Gemini key

This release fixes a failover bug found by testing against the real
provider APIs.

Fixed

• A bad Gemini API key now falls back to the next provider. Gemini
  reports a bad key as a 400 error, which ModelSwitch treated as a
  problem with the request itself, so it stopped without trying other
  providers. It now reads the specific reason Gemini gives and moves on
  to the next model in the alias.

Other 400 errors still stop the request, so a prompt one provider
refuses isn't sent on to the others. Records still show Gemini's real
400, now with the code API_KEY_INVALID.

Upgrading

No database migration or config changes are needed.

This is a pre-1.0 release, so details may change.