Fix model header normalization for custom providers

[corazzione]

Fixes #1531

Summary

This PR makes model construction more tolerant of custom provider headers that are accidentally declared at the model root in config.yaml.

Problem

Some custom-provider configs include HTTP headers such as User-Agent. When those header entries leak into the model config root instead of being nested under default_headers, DeerFlow forwards them as direct constructor kwargs and provider SDKs fail with errors like:

• AsyncCompletions.create() got an unexpected keyword argument 'User-Agent'

Fix

• normalize header-like extra config keys into default_headers before instantiating the model class
• preserve the existing default_headers flow
• add a regression test covering User-Agent normalization for a custom provider config

Validation

• ./.venv/Scripts/python.exe -m pytest tests/test_model_factory.py -q
• ./.venv/Scripts/python.exe -m ruff check packages/harness/deerflow/models/factory.py tests/test_model_factory.py