fix: handle None on_delete in describe handler

[foogunlana]

Summary

• Fix AttributeError when field.remote_field.on_delete is None (e.g., M2M through relations)
• Use getattr with a None check instead of hasattr to safely access on_delete

Fixes #78

Changes

In _get_field_metadata (django_admin_mcp/handlers/meta.py):

# Before
if hasattr(field.remote_field, "on_delete"):
    metadata["on_delete"] = field.remote_field.on_delete.__name__

# After
on_delete = getattr(field.remote_field, "on_delete", None)
if on_delete is not None:
    metadata["on_delete"] = on_delete.__name__

Test plan

• Verify describe handler works for models with M2M through relations
• Verify describe handler still correctly reports on_delete for ForeignKey fields
• Verify no regression for models without relationship fields

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