[r4victor]

Follow-up to #4418, which checked src/tests/_internal/core. Only tests and the pyright config change.

• Check all of src/tests with pyright.
• Narrow optional and union values in tests with asserts, isinstance, and get_or_error, adding shared get_verified_preset and get_fake_api helpers for the CLI preset tests.
• Construct shorthand test inputs with model_validate and expected values with typed constructors.
• Use replicas= and groups= instead of the legacy count= and list-valued replicas= in replica group tests.
• Build AWS volumes with their configuration instead of mutating the volume configuration union.
• Reach the optional fluentbit classes through the module instead of importing them conditionally.
• Compare proxy repo listings as sorted lists, since pyright doesn't see frozen pydantic models as hashable.
• Mock API clients with MagicMock in the run collection tests instead of hand-written fakes.
• Use httpx2 in the model proxy test, as the installed openai client expects.
• Ignore POSIX-only names in tests when pyright checks them for Windows.