fix: align KafkaError stub keyword with runtime

[wgu9 (wgu9)]

Fixes #2243.

Changes

• Rename the first KafkaError stub initializer parameter from code to error, matching the C extension keyword accepted at runtime.
• Add a regression test covering the runtime keyword contract: error= is accepted and code= raises TypeError.

Validation

• MYPYPATH=src uvx --from mypy==1.18.2 mypy --python-version 3.11 -c $'from confluent_kafka import KafkaError\\nKafkaError(error=1)'\n- MYPYPATH=src uvx --from mypy==1.18.2 mypy --python-version 3.11 -c $'from confluent_kafka import KafkaError\\nKafkaError(code=1)' confirms code= is rejected\n- python3 -m py_compile tests/test_kafka_error.py\n- uvx --from black black --check --diff tests/test_kafka_error.py\n- git diff --check\n\nNote: I could not run pytest tests/test_kafka_error.py locally because this fresh checkout does not have the compiled confluent_kafka extension or pytest installed. The added pytest is intended for CI, where the package is built before test execution.

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