fix(api-core): handle list-shaped REST error payloads

[cyphercodes]

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Fixes #18223 🦕

Summary

• Normalize list-shaped REST error payloads to the first dict item before reading the standard error object.
• Preserve existing dict payload behavior and fall back to unknown error when no dict item is present.
• Add unit coverage for list-wrapped error payloads.

Tests

• pytest -q tests/unit/test_exceptions.py -k 'http_response or error_details_from_rest_response'
• pytest -q tests/unit/test_exceptions.py
• ruff format --check --target-version=py310 --line-length=88 google/api_core/exceptions.py tests/unit/test_exceptions.py
• flake8 google/api_core/exceptions.py tests/unit/test_exceptions.py
• git diff --check

CLA

Google CLA is required by the repository; awaiting the repository's CLA check on this PR.

[gemini-code-assist]

Code Review

This pull request updates the format_http_response_error function in exceptions.py to handle cases where the HTTP response payload is a list, extracting the first dictionary item from the list to parse the error. It also adds corresponding unit tests. The reviewer suggested ensuring that payload is explicitly validated as a dictionary to prevent potential AttributeError crashes if the payload is of another non-dictionary type, which aligns with defensive programming guidelines.

[gemini-code-assist]

To ensure robust defensive programming against untrusted or malformed REST payloads, we should guarantee that payload is a dictionary before calling .get() on it. If the payload is a string, number, or other non-dictionary type, the current implementation will raise an AttributeError. Replacing the falsy check with an explicit isinstance(payload, dict) check prevents potential crashes.

Suggested change

	if isinstance(payload, list):
	payload = next((item for item in payload if isinstance(item, dict)), {})
	payload = {} if not payload else payload
	if isinstance(payload, list):
	payload = next((item for item in payload if isinstance(item, dict)), {})
	if not isinstance(payload, dict):
	payload = {}