Remove underlyingError from GenerateContentError.invalidAPIKey

[andrewheard]

Removed the underlyingError: Error associated value from the GenerateContentError.invalidAPIKey enum case. We could consider adding a specific value (similar to RPCError but public) in the future but for now the enum case itself (invalidAPIKey) captures the information that devs need to act on.

Same expected usage as before:

do {
  let content = try await model.generateContent("Why is the sky blue?")
} catch GenerateContentError.invalidAPIKey {
  // Devs can decide how they want to display this error to users
  print("The provided API key is invalid.")
} catch {
  // A different error occurred
}