[ty] Improve diagnostic for failed assignment to a `Callable` type. (#25308)
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## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
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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ruff
a8aceb1a - [ty] Improve diagnostic for failed assignment to a `Callable` type. (#25308)
