Throw GenerateContentError.invalidAPIKey in generateContentStream

[andrewheard]

• Fixed generateContentStream to throw GenerateContentError.invalidAPIKey instead of GenerateContentError.internalError
  • Now matches the generateContent behaviour (changed in Add invalidAPIKey case to GenerateContentError #91 / Remove underlyingError from GenerateContentError.invalidAPIKey #92)
• Added a unit test for this scenario
  • Note: Uses the same unary-failure-api-key.json sample response as the generateContent test since the backend response is the same for both unary and streaming requests