refactor(python): express param strictness as a type, not a flag

Replace the relaxStrict boolean + shouldRelaxStrictParameterTyping hook with
an explicit PydanticCoercibleType subclass of PydanticType that never emits
Pydantic strict types for the scalar kinds where strictness blocks coercion.

Selection moves into a getPydanticParameterType() factory: PythonFastAPIServer
returns PydanticCoercibleType for path/query/header params and the strict base
type for everything else. The rule ("wire-string params are never strict") is
now a code structure rather than a flag callers must remember to set, and the
explanatory comment lives in one place on the class it describes.

Behaviour-preserving: the regenerated python-fastapi sample is byte-identical
to the previous commit. Python codegen tests (fastapi, client) pass.

Claude-Session: https://claude.ai/code/session_019g7iwAyg7ErX1WrhqHyTn6

Author: Goopher