fix(security): warn on empty allowed_hosts and improve 421/403 response bodies #2307

Job	Run time
checks / pre-commit	33s
checks / readme-snippets	16s
checks / test (3.10, locked, windows-latest)	2m 14s
checks / test (3.13, locked, windows-latest)	2m 33s
checks / test (3.12, lowest-direct, ubuntu-latest)	59s
checks / test (3.13, lowest-direct, ubuntu-latest)	1m 1s
checks / test (3.11, lowest-direct, windows-latest)	2m 51s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 17s
checks / test (3.14, locked, windows-latest)	3m 50s
checks / test (3.14, lowest-direct, windows-latest)	2m 1s
checks / test (3.14, locked, ubuntu-latest)	2m 20s
checks / test (3.10, lowest-direct, windows-latest)	3m 8s
checks / test (3.13, locked, ubuntu-latest)	47s
checks / test (3.10, lowest-direct, ubuntu-latest)	56s
checks / test (3.12, locked, ubuntu-latest)	59s
checks / test (3.12, lowest-direct, windows-latest)	3m 22s
checks / test (3.11, locked, windows-latest)	6m 20s
checks / test (3.11, lowest-direct, ubuntu-latest)	52s
checks / test (3.13, lowest-direct, windows-latest)	2m 50s
checks / test (3.11, locked, ubuntu-latest)	52s
checks / test (3.10, locked, ubuntu-latest)	55s
checks / test (3.12, locked, windows-latest)	3m 38s
all-green	3s
	44m 37s

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