fix: Handle explicit null values in SessionResetPolicy config (#1119)

[Bartok9]

Summary

When users explicitly set at_hour or idle_minutes to null in their config.yaml, the gateway previously failed with a TypeError. This PR fixes that by properly handling null values in SessionResetPolicy.from_dict().

Problem

Python's dict.get() only returns the default value when a key is missing, not when the key exists with a None/null value:

default_reset_policy:
  mode: both
  at_hour: null      # This causes TypeError
  idle_minutes: null # This too

Solution

Explicitly check for None values before applying defaults:

at_hour = data.get("at_hour")
if at_hour is None:
    at_hour = 4

Changes

• gateway/config.py: Update SessionResetPolicy.from_dict() to handle null values
• tests/gateway/test_config.py: Add regression tests for null value handling

Testing

All existing tests pass, plus new tests for:

• Explicit null values → use defaults
• Partial null values → defaults for null, preserve set values
• Empty dict → all defaults

Fixes #1119

[Bartok9]

Closing as superseded by #1194, which merged the same null handling fix along with the API timeout feature. Thanks!