Demote `WARN` about `format=` to `INFO`, and/or make analysis more precise.

[nrfulton]

This message creates a lot of noise when issued at the WARN level:

=== 18:00:43-WARNING ======
Mellea assumes you are NOT using the OpenAI platform, and that other model providers have less strict requirements on support JSON schemas passed into `format=`. If you encounter a server-side error following this message, then you found an exception to this assumption. Please open an issue at github.com/generative_computing/mellea with this stack trace and your inference engine / model provider.

• Fix grammar in the message.
• Demote to INFO, or
• make the analysis more precise by only emitting the message when (1) the grammar used to constrain the generation is not a grammar supported by the OpenAI platform, and (2) the OpenAI platform is being used.