Bug: copying or merging CaseInsensitiveDict (event.headers) loses case-insensitive lookup #8536

Expected Behaviour

A copy or merge of event.headers should still be a CaseInsensitiveDict, so that headers.copy().get("Content-Type") and (headers | {"X-Trace-Id": "1"})["x-trace-id"] work the same way they do on event.headers itself.

Current Behaviour

CaseInsensitiveDict overrides get, update, __getitem__ and friends, but not copy, __or__, __ror__ or __ior__, so those fall through to dict:

• copy() and | return a plain dict. The stored keys are already lowercased, so looking up the original header name on the result returns None.
• |= goes through dict's own update path rather than the overridden update, so the new key keeps its case and a lowercase lookup misses it.

This came up after #8517: a handler that copies the headers to add one before forwarding them loses the case-insensitive lookup on the copy.

Code snippet

from aws_lambda_powertools.utilities.data_classes import APIGatewayProxyEvent

event = APIGatewayProxyEvent(
    {"httpMethod": "GET", "path": "/", "headers": {"Content-Type": "application/json"}, "requestContext": {}}
)

event.headers.get("Content-Type")
# 'application/json'

event.headers.copy().get("Content-Type")
# None, and type(...) is dict

(event.headers | {"X-Trace-Id": "1"}).get("x-trace-id")
# None

headers = event.headers
headers |= {"X-Trace-Id": "1"}
list(headers)
# ['content-type', 'X-Trace-Id']

Possible Solution

Override copy, __or__, __ror__ and __ior__ on CaseInsensitiveDict so they return a CaseInsensitiveDict and route through update. Happy to open a PR with tests.

Steps to Reproduce

Run the snippet above against develop.

Powertools for AWS Lambda (Python) version

latest (develop at 5b74209)

AWS Lambda function runtime

3.12

Packaging format used

PyPi

Debugging logs

type(event.headers.copy())
# <class 'dict'>