fix: provide helpful error when table heading is not configured

[dimitri-yatsenko]

Summary

Provide helpful error messages when using tables from non-activated schemas.

Problem

When using a table from a non-activated schema, operations produced confusing errors:

schema = dj.Schema()  # Not activated (no database name)

@schema
class TableA(dj.Manual):
    definition = "..."

TableA()  # AttributeError: 'NoneType' object has no attribute 'non_blobs'
TableA().primary_key  # AttributeError: 'NoneType' object has no attribute 'primary_key'

Solution

Override the heading property in Table class to check for None and raise a clear error:

@property
def heading(self):
    if self._heading is None:
        raise DataJointError(
            f"Table `{self.__class__.__name__}` is not properly configured. "
            "Ensure the schema is activated before using the table. "
            "Example: schema.activate('database_name') or schema = dj.Schema('database_name')"
        )
    return self._heading

Before/After

Operation	Before	After
table.heading	AttributeError: 'NoneType'...	DataJointError: Table X is not properly configured...
table.primary_key	AttributeError: 'NoneType'...	DataJointError: Table X is not properly configured...

Test plan

• Added regression test for non-activated schema heading error

Closes #1039

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