lowlevel Server: widen on_* return types for InputRequiredResult; add subscriptions/listen slot #2959

Job	Run time
checks / readme-snippets	15s
checks / pre-commit	40s
checks / test (3.11, lowest-direct, ubuntu-latest)	1m 1s
checks / test (3.14, lowest-direct, windows-latest)	1m 32s
checks / test (3.11, lowest-direct, windows-latest)	1m 41s
checks / test (3.13, locked, windows-latest)	1m 25s
checks / test (3.14, locked, ubuntu-latest)	2m 2s
checks / test (3.13, lowest-direct, ubuntu-latest)	1m 5s
checks / test (3.10, lowest-direct, ubuntu-latest)	1m 9s
checks / test (3.10, locked, windows-latest)	1m 38s
checks / test (3.10, locked, ubuntu-latest)	1m 2s
checks / test (3.11, locked, windows-latest)	1m 41s
checks / test (3.11, locked, ubuntu-latest)	1m 3s
checks / test (3.12, locked, ubuntu-latest)	1m 3s
checks / test (3.13, locked, ubuntu-latest)	53s
checks / test (3.12, locked, windows-latest)	1m 24s
checks / test (3.13, lowest-direct, windows-latest)	1m 35s
checks / test (3.14, lowest-direct, ubuntu-latest)	59s
checks / test (3.10, lowest-direct, windows-latest)	1m 28s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 14s
checks / test (3.12, lowest-direct, windows-latest)	1m 28s
checks / test (3.14, locked, windows-latest)	2m 22s
all-green	3s
	28m 43s

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