Do not validate model requests and responses

[jvstme]

Problem

Currently, dstack's OpenAI-compatible endpoint performs strict validation of all requests and responses against a hardcoded schema. However, some model launchers don't adhere to the official OpenAI schema, and the official schema can also evolve rapidly. As a result, some models or launchers can't be used with dstack because validation fails for them. Examples: #1777, #2228.

Solution

Do not perform strict validation for format: openai models. Only extract the fields that dstack needs, such as the model field, and pass the rest of the fields in requests and responses as is.

Note: strict validation may still be necessary for format: tgi models in order to perform format conversion.