[maharshi-coding]

Fixes #5333

What was going wrong?

When narrowing an attribute that doesn't exist, Pyrefly was caching its type as object instead of Unknown.
For example:

x: int = 10

if x.foo:
    print(x.foo[0])

Pyrefly correctly reports that int has no attribute foo, but it also reports another error when trying to index x.foo. That second error isn't really useful since the missing attribute was already reported.

What I changed

I made two changes to fix this:

• attr.rs: Changed how missing attributes are handled during narrowing. They now produce an error Unknown instead of falling back to object. Actual internal lookup errors still use the existing object fallback.
• expr.rs: Adjusted the error-origin check so that reading a previously cached error type doesn't trigger another diagnostic.

This also preserves valid types when narrowing unions. For example, if one union member has the attribute and another doesn't, the result can now be Foo | Unknown instead of Foo | object.

Testing

Added 8 regression tests and updated one existing expectation.
The tests cover missing attributes in different conditions, including truthy checks, is not None, nested branches, and len() checks. I also checked union types, class attributes, isinstance() narrowing, and error provenance diagnostics.
The main thing I wanted to make sure of was that Pyrefly still reports the original missing-attribute error without producing additional errors from the same issue, while keeping valid narrowing behavior unchanged.

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[meta-codesync]

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