fix(api-core): address PR review comments for requests.py and test_requests

[hebaalazzeh]

Description

This PR addresses code review feedback regarding edge cases and test coverage in requests.py and test_requests.py:

1. requests.py: Updated fallback logic for proto-plus messages/other objects when is_proto3_optional=True from if not getattr(...) to if getattr(...) is None. This distinguishes missing fields from fields explicitly set to empty values (e.g., ""), matching the dictionary handling behavior.
2. test_requests.py: Removed the unused __contains__ mock helper methods from MockRequest and MockValueErrorRequest since the helper uses the in operator only for dictionaries.
3. test_requests.py: Added parameterized test cases for is_proto3_optional=True with explicit empty string values "" to verify they are preserved and not overwritten.
4. test_requests.py: Changed test assertions to use re.fullmatch instead of re.match to guarantee strict UUID format validation.

[gemini-code-assist]

Code Review

This pull request modifies setup_request_id to check if a field is explicitly None rather than falsy (such as an empty string) when handling proto-plus or other objects, preventing an empty string from being overwritten by a new UUID. Corresponding test cases for empty strings were added, unused mock methods were removed, and re.match was updated to re.fullmatch in the tests. The reviewer suggests extending the test coverage by adding a test case for MockProtoRequest with an empty string to ensure standard protobuf messages are also verified.

[gemini-code-assist]

While adding a test case for MockRequest with an explicit empty string when is_proto3_optional=True is great, we should also add a corresponding test case for MockProtoRequest to ensure standard protobuf messages (which implement HasField) are also covered and behave correctly under this scenario. Please consider adding (MockProtoRequest(request_id=""), True, "") and the corresponding ID "proto3_optional_explicit_empty_proto" to the ids list.