Fix JsonType to accept Mapping[str, Any] instead of recursive Mapping[str, "JsonType"]

[driphtyio]

Fixes #7443

Problem

The recursive JsonType definition used Mapping[str, "JsonType"], which causes mypy to reject valid dict subtypes like dict[str, object] or dict[str, Collection[str]] due to mypy's invariance handling of recursive Mapping types.

Reproduction

def fn(d: dict[str, str]) -> None:
    j = {"foo": d, "bar": "hi"}
    requests.post("https://example.com", json=j)  # mypy error

Fix

Replaced the recursive Mapping[str, "JsonType"] with Mapping[str, Any]. This matches how Python's json.dumps actually accepts any serializable value. Any is the standard pattern used by CPython's json module stubs for the same purpose.

Testing

• 407 relevant tests pass (all pre-existing failures are network-dependent)
• No new mypy errors introduced
• pyright and ty continue to work correctly