fix(gateway): handle explicit null values in SessionResetPolicy config

[eren-karakus0]

Summary

Fixes TypeError when users set at_hour or idle_minutes to null in their config.yaml.

dict.get("key", default) only returns the default when the key is missing, not when it exists with a None value. This caused None to reach the validation logic (0 <= None <= 23), raising a TypeError and crashing the gateway on startup.

Changes

• gateway/config.py: Replace data.get("at_hour", 4) pattern with explicit None checks in SessionResetPolicy.from_dict()
• tests/gateway/test_config.py: Add 3 test cases — explicit null values, missing keys, partial null

Test plan

• Explicit null for both at_hour and idle_minutes → defaults applied
• Missing keys → defaults applied
• One field null, the other valid → only null field gets default
• Existing roundtrip test still passes

Fixes #1119

[teknium1]

Thanks for the contribution! This fix is already on main — the null-coalescing for at_hour and idle_minutes was merged independently, and mode was fixed in #1488. There's also an existing test (test_from_dict_treats_null_values_as_defaults) covering the null-value case. Closing as redundant.