Bug: exception handlers hijack the default 404 and bypass _to_response (body null on ALB, dict/tuple crash) #8579

Expected Behaviour

• An unmatched route returns 404 even when a broad @app.exception_handler(Exception) is registered.
• Whatever an exception handler returns goes through the same _to_response adaptation as a route return value, so Response(body=None) on ALB becomes "" and a dict/tuple is converted or rejected clearly.

Current Behaviour

All in api_gateway.py, confirmed through a real API Gateway REST API:

1. 404 hijacked. not_found_handler (line 2847) synthesizes a NotFoundError() that was never raised and looks it up with lookup_exception_handler, which walks the MRO. With @app.exception_handler(Exception), which the docs suggest as a catch-all, every unmatched route returns 500: GET /nope → 500 generic handler got NotFoundError().
2. body: null on ALB. _call_exception_handler (line 2952) wraps handler(exp) directly in the response builder. On ALBResolver, a handler returning Response(status_code=400, body=None) emits "body": null and ALB answers 502. The same Response returned from a route emits "" because _to_response fixes it.
3. dict/tuple return crashes. A handler returning {"error": str(exc)}, 400 raises AttributeError: 'tuple' object has no attribute 'is_json', masking the original exception. Real API: 502.

Code snippet

from aws_lambda_powertools.event_handler import ALBResolver, APIGatewayRestResolver, Response
from tests.functional.utils import load_event

ev = load_event("apiGatewayProxyEvent.json")  # GET /my/path

app = APIGatewayRestResolver()

@app.exception_handler(Exception)
def generic(exc):
    return Response(500, "text/plain", f"generic got {exc!r}")

@app.get("/other")
def other():
    return {}

print(app.resolve(ev, None))  # statusCode 500, body "generic got NotFoundError()"  (expected 404)

alb = ALBResolver()

@alb.exception_handler(ValueError)
def h(exc):
    return Response(status_code=400, content_type=None, body=None)

@alb.get("/lambda")
def r():
    raise ValueError("x")

print(alb.resolve(load_event("albEvent.json"), None)["body"])  # None  (expected "")

app2 = APIGatewayRestResolver()

@app2.exception_handler(ValueError)
def h2(exc):
    return {"error": str(exc)}, 400

@app2.get("/my/path")
def r2():
    raise ValueError("boom")

app2.resolve(ev, None)  # AttributeError: 'tuple' object has no attribute 'is_json'

Possible Solution

In not_found_handler, look up only an exact NotFoundError registration instead of walking the MRO. In _call_exception_handler, wrap the handler result with self._to_response(handler(exp)).

Steps to Reproduce

Run the snippet above with the tests/events fixtures. Full context in #8576.

Powertools for AWS Lambda (Python) version

latest

AWS Lambda function runtime

3.13

Packaging format used

PyPi

Debugging logs