Fix KeyError in _EncodeCustomFieldNames when field name is a substring

[YonathanZetune]

Problem

_EncodeCustomFieldNames previously checked if python_name in encoded_value: (substring membership in the raw serialized JSON str) instead of checking key membership in decoded_value (dict).

When a message has custom JSON field mappings registered via AddCustomJsonFieldMapping and an unset field's python_name appears as a substring of either:

1. Another populated field name (e.g., service_directory_name vs. metadata_service_directory_name), or
2. Any string field value in the JSON payload,

python_name in encoded_value evaluates to True, causing decoded_value.pop(python_name) to raise a KeyError.

Solution

Check if python_name in decoded_value: so key membership is verified against the decoded dictionary, matching _DecodeCustomFieldNames. Added unit test coverage for both field-name and field-value substring collisions.