fix: preserve known exports in dynamic __all__

[nightcityblade]

Summary

Preserve statically known __all__ names when a later dynamic append or extend cannot be resolved. This prevents explicit re-exports such as torch.Tensor from being reported as implicit while keeping the conservative fallback for the unresolved portion of __all__.

Fixes #4424

Test Plan

• cargo test -p pyrefly test::imports:: (126 passed)
• cargo clippy --release -p pyrefly --all-targets (passed; two unrelated existing warnings)
• cargo fmt -- --check
• Built Pyrefly and checked from torch import Tensor against torch 2.10.0 with --preset all (0 errors)

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