Add unit tests for PostgresSchemaInspector

[NoMo-101]

`tests/test_inspector.py` currently exists but is empty. We need real unit test coverage for `PostgresSchemaInspector` now that the db layer has been refactored to be database-agnostic (see models.py, interfaces.py, postgres_connector.py, postgres_inspector.py).

What to test

Three methods on `PostgresSchemaInspector`:

• `fetch_tables()` — returns `list[TableInfo]`
• `fetch_columns()` — returns `list[ColumnInfo]`, including the `is_nullable` string-to-bool conversion (`'YES'`/`'NO'` → `True`/`False`)
• `fetch_foreign_keys()` — returns `list[ForeignKeyInfo]`

Approach

`PostgresSchemaInspector.init` takes anything implementing `DBConnectorInterface` — not necessarily a real `PostgresConnector`. Rather than testing against a live Postgres database, build a `MockConnector` that implements `DBConnectorInterface` and returns hardcoded rows from `execute_query()`. This lets tests run without any real database connection.

Example shape:

class MockConnector(DBConnectorInterface):
    def __init__(self, fake_rows):
        self.fake_rows = fake_rows
    def execute_query(self, sql, params=None):
        return self.fake_rows
    def connect(self): return True
    def disconnect(self): pass
    @property
    def is_connected(self): return True

Then feed `PostgresSchemaInspector` a `MockConnector` with known fake rows, and assert the returned dataclass objects have the correct fields.

What "done" looks like

• At least one test per method (`fetch_tables`, `fetch_columns`, `fetch_foreign_keys`)
• A test specifically confirming `is_nullable` converts `'YES'` → `True` and `'NO'` → `False`
• Tests run with pytest and don't require a live database connection