Detect redundant cast usage and emit warning

[sanma613]

Summary

This PR adds detection for redundant usage of typing.cast where the value being cast already matches the target type. In such cases, a warning is now emitted to inform users of unnecessary casting.

Implementation Details

• A check was added before applying the cast to compare the inferred type of the value against the target type.
• If both types match, an ErrorKind::RedundantCast warning is emitted instead of applying the cast.
• The cast still proceeds (non-fatal) to maintain soundness.
• Includes tests under redundant_cast.rs.

Example

from typing import cast

def f(x: int) -> int:
    return cast(int, x)  # warning: Redundant cast

Testing

• Added test case: test_redundant_cast_simple
• Added test case: test_redundant_cast_with_annotation
• Added test case: test_non_redundant_cast

Closes #593

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[yangdanny97]

I don't think subtype is a function?

Try is_subset_eq

[yangdanny97]

this message is slightly misleading if the thing being cast is Any

why not something simpler like "redundant cast to {}"? note that when we pass the type we should call .deterministic_printing() on it

[yangdanny97]

these messages don't seem to match the implementaiton in special_calls.rs

[yangdanny97]

In addition to other comments, don't forget to run cargo fmt and cargo clippy

Thanks!