[ty] Improve diagnostic for failed assignment to a `Callable` type. (#25308)

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## Summary

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This improves the diagnostic we show when an assignment to a `Callable`
type fails. That diagnostic now contains additional context that
describes the inferred callable type that failed. The result is that the
full explanation of the failed assignment is more coherent.

Closes https://github.com/astral-sh/ty/issues/866.

## Test Plan

Please see updated tests.
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