[Firebase AI] Fix integration tests and re-enable Dev API configs

[andrewheard]

• Updated testCountTokens_functionCalling to check that the token count is > 0.
  • This will make the test more resilient to backend changes in token counting.
• Re-enabled Developer API configs since the backend issues have been resolved.
• Disabled Vertex AI configs for generateImage since the backend returns empty SafetyRatings in the array, which breaks our decoding.
  • Note 1: This behaviour does not appear to occur with production models -- this test uses the experimental Gemini 2.0 Flash model with Image Generation support (gemini-2.0-flash-exp).
  • Note 2: This issue is not observed on Google AI (Developer API).

#no-changelog

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