test: remove duplicate Discovery class that shadowed test_discovery_http_is_closed

[Tatamis]

Fixes #2757.

tests/test_discovery.py defined class Discovery(unittest.TestCase) twice: once near line 498 (added in #1038 to test that build() closes the httplib2.Http it creates internally) and once further down (pre-existing). In Python, the second class definition silently overwrites the first at import time, so test_discovery_http_is_closed was never collected by pytest and has been dead code since 2020.

The dead test was also broken on its own terms: HttpMock.close is a plain method, not a mock object, so calling .assert_called_once() on it raises AttributeError rather than testing anything — confirmed by isolating and running it directly.

Fix

Removes the duplicate class and moves a corrected version of the test into the real Discovery class, patching httplib2.Http directly so the assertion actually exercises the close-the-internal-http-client behavior in discovery.py (the discovery_http.close() call made when build() creates its own http client to fetch the discovery document).

Verified the new test both passes against current code and fails (as expected) if that discovery_http.close() call is removed.

Test plan

• pytest tests/test_discovery.py — 167 passed (up from 166), same 5 pre-existing unrelated failures (credentials/cache/mtls tests that fail in this sandboxed environment regardless of this change).
• black --check clean.

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[gemini-code-assist]

Code Review

This pull request refactors and relocates the test_discovery_http_is_closed test in tests/test_discovery.py to use @mock.patch for mocking httplib2.Http instead of relying on HttpMock. The feedback suggests two improvements: first, to mock the response content as bytes by reading the datafile with 'rb' to better align with Python 3 behavior; second, to use a mock with a proper spec for credentials instead of a generic MagicMock to ensure type-checking compatibility.