fix(mixins): Add headers and timeout to FindAllChildrenCommandMixin …

[telesoho]

Summary

This PR fixes unauthorized errors when listing child resources (e.g. plan charges) through the Python client.

In FindAllChildrenCommandMixin.find_all() (lago_python_client/mixins.py), the GET request was sent without authentication headers.
As a result, endpoints like GET /api/v1/plans/{code}/charges could return 401 Unauthorized even when the API key was valid for other plan operations.

This change adds:

• headers=make_headers(api_key=self.api_key)
• timeout=timetour

to the child find_all GET request, aligning it with other authenticated client calls.

Why

plan find/update calls were successful, but child collection fetches failed with 401 due to missing Bearer auth headers in this specific mixin path.

Testing

• Ran test suite: python3 -m pytest tests
• Verified no linter/type issues introduced in the modified file.
• Manually validated that child collection requests now include auth headers and no longer fail with unauthorized responses under valid credentials.

Labels

• bug
• improvement

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[cla-check-bot]

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