fix: return 400 for invalid webhook configuration

[suguanYang]

Summary

• WebhookConfigException was storing details as None, so POST /v2/jobs with an invalid webhook (for example a localhost URL) crashed in the exception handler and returned 500 UnknownException instead of 400.
• Pass details through to KnowhereException, and read retry_after from exc.details or {} so a missing details dict cannot turn a client error into a server error.
• Add a contract test that raises the same webhook configuration exception and expects 400 INVALID_ARGUMENT.

Test plan

• uv run pytest tests/contract/test_exception_handlers_contract.py::test_invalid_webhook_config_returns_client_error
• After deploy, confirm a POST /v2/jobs with a localhost webhook returns 400 Invalid webhook URL and does not emit UnknownException in Logfire.

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