[Siyet]

Part of #3 (step 1).

• A body without a Content-Type header follows FastAPI's strict_content_type: 415 by
  default, JSON when the setting is False. A Content-Type header with an empty media
  type, such as ; charset=utf-8, is rejected with 415.
• Deeply nested JSON returns 400 instead of 500.
• A NaN or infinite float in a typed response is encoded as null. When the JSON round
  trip of a response fails validation, the Python values are validated instead, so the
  result for values that pass the round trip does not change. If that check finds a
  validation error, that error is reported, because it points at the actual problem.
• Body validation errors (422) are raised after dependencies, as in FastAPI. Malformed or
  too deeply nested JSON (400) and a missing or non-JSON content type (415) are rejected
  before dependencies. When parameters are also invalid, only the parameter errors are
  reported.
• -> None declares no response model, as in FastAPI.
• Form() and File() parameters are left to FastAPI; a Struct in such a parameter is
  rejected at registration with a clear error.
• A CI workflow runs the tests on Python 3.11 to 3.14 with the latest and the lowest
  supported FastAPI and msgspec, and checks the built distributions.

The README describes the new behavior.