Fix error result message fallback

[sanjitrajendiran42]

Summary

• prefer the CLI result text when an error result has an empty errors list
• keep the existing subtype fallback for minimal/older result payloads
• add regression coverage for API-style error results with subtype=success

Fixes #1031.

Tests

• .venv/bin/python -m pytest tests/test_query.py::TestProcessExitAfterErrorResult
• .venv/bin/python -m ruff check src/claude_agent_sdk/_internal/query.py tests/test_query.py
• .venv/bin/python -m ruff format --check src/claude_agent_sdk/_internal/query.py tests/test_query.py
• .venv/bin/python -m pytest tests/test_query.py
• .venv/bin/python -m mypy src/