Make Cursor an iterator conforming to DB-API 2.0

[Copilot]

This PR updates the Cursor classes to fully support the standard Python iterator protocol, aligning the behavior with PEP-249 and parity with PyMySQL (referencing PyMySQL#995).

Changes made

• Implemented Iterator Protocol in BaseCursor: Added __iter__ to return self and __next__ to yield rows using fetchone(), raising StopIteration when the result set is exhausted.
• Cleaned up MixIns: Removed redundant and non-standard __iter__ and next() override implementations from CursorStoreResultMixIn and CursorUseResultMixIn.
• Added Tests: Verified iterator compatibility for both Cursor and SSCursor.

Example usage

cursor.execute("SELECT id, name FROM test_table")

# The cursor is now a direct iterator
assert iter(cursor) is cursor

# Can be used directly in loops
for row in cursor:
    print(row)