Fix ASGI middleware assignment typing

[hansu650]

Fixes #438.

The documented app.asgi_app = middleware(app.asgi_app) pattern was treated by
type checkers as assignment to a method. This keeps the existing runtime method
unchanged while exposing an assignable ASGI application callable during static
type checking.

The existing middleware test now verifies the assignment without a type-ignore,
and the original OpenTelemetry example from the issue passes mypy.

Validation:

• pytest: 256 passed
• mypy on the OpenTelemetry reproduction: no issues found
• Ruff check and format: passed
• pre-commit: passed
• git diff --check: passed

Full-project mypy on Windows retains one existing readline.set_completer
platform error; no new errors are introduced by this change.

[davidism]

https://palletsprojects.com/contributing/llm-ai