fix: address several open SDK bugs (#2937, #2938, #3007, #2991, #2995) - #1

Fixes the open upstream bugs in googleapis/python-genai that can be fixed in the SDK. The remaining open bugs (googleapis#3003, googleapis#2974, googleapis#2936, googleapis#2935, googleapis#2927, googleapis#2926 and googleapis#2925) are server-side, model-behavior or API-docs issues.

Changes

• FinishReason enum is missing MALFORMED_RESPONSE, which the API actually returns googleapis/python-genai#2937: add FinishReason.MALFORMED_RESPONSE, which the API already returns.
• Async file upload retry blocks the event loop googleapis/python-genai#2938: the httpx branch of _async_upload_fd used time.sleep for its retry backoff, which blocked the event loop. It now uses await asyncio.sleep, like the aiohttp branch already does.
• response_schema silently drops per-field descriptions on nested Pydantic models when resolving $ref googleapis/python-genai#3007: when process_schema resolved a $ref, annotations next to it were overwritten by the referenced definition. For example, two fields of the same nested model with different Field(description=...) both ended up with the model's docstring. Now the field's own annotations win, and the model's docstring is used only when the field has no description.
• v2.18 defers building Pydantic models until first use, which causes errors in a multithreaded context googleapis/python-genai#2991: defer_build=True is not thread safe. Several threads using a model for the first time at once could see it half-built (AttributeError: ... no attribute '__pydantic_core_schema__'). _common.BaseModel now builds the model under a lock in __init__ and model_validate*. These overrides are hidden from type checkers, so the signatures Pydantic generates are unchanged.
• BaseApiClient.__del__ schedules aclose() on whatever loop is current; on Python 3.14 a cross-loop close leaves the aiohttp connector's _wait_for_close task destroyed while pending googleapis/python-genai#2995: in __del__, each aiohttp session is now closed on the event loop that created it (with call_soon_threadsafe when that loop runs in another thread), and the scheduled close tasks are kept referenced so they are not destroyed while pending. The same change applies to AsyncHttpxClient.__del__.

Testing

• New tests for each fix, in test_types.py, test_upload_errors.py, test_schema.py, test_common.py and test_client_close.py.
• pytest google/genai/tests/{types,common,transformers,client}: 633 passed, 6 skipped, 1 failed. The failure is test_aiohttp_retries_failed_request_retries_unsuccessfully_mtls, which fails the same way on main without these changes: it gets a real 404 from the network in this sandbox.
• v2.18 defers building Pydantic models until first use, which causes errors in a multithreaded context googleapis/python-genai#2991 reproduction (16 threads creating a GenerateContentConfig at once, 100 separate processes): 19/100 runs failed on main, 0/100 with this change.
• BaseApiClient.__del__ schedules aclose() on whatever loop is current; on Python 3.14 a cross-loop close leaves the aiohttp connector's _wait_for_close task destroyed while pending googleapis/python-genai#2995 could not be reproduced on Python 3.11: main already closes the session in that setup, and the issue was reported on 1.75.0 with Python 3.14. The change is defensive hardening, not a verified fix.

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