[match-case] fix matching against typing.Callable and Protocol types.

[randolf-scholz]

• Fixes match-case class checking against collection.abc.Callable raises error [misc] #14014
• Addresses match-case part of [match-case]/[isinstance] narrowing logic against the special AnyCallable Protocol differs from callable() check #19470

Added extra logic in checker.py:conditional_types function to deal with structural types such as typing.Callable or protocols.

new tests

• testMatchClassPatternCallable: tests case Callable() as fn usage
• testMatchClassPatternProtocol: tests case Proto() usage, where Proto is a Protocol
• testMatchClassPatternCallbackProtocol: tests case Proto() usage, where Proto is a Callback-Protocol
• testGenericAliasIsinstanceUnreachable: derived from a mypy-primer failure in mesonbuild. Tests that isinstance(x, Proto) can produce unreachable error.
• testGenericAliasRedundantExprCompoundIfExpr: derived from a CI failure of python runtest.py self of an earlier version of this PR.

modified tests

• testOverloadOnProtocol added annotations to overload implementation, which wasn't getting checked. Added missing return. Fixed return type in second branch.

[sterliakov]

Maybe this line shouldn't be separated from the previous one? Now it's a bit ambiguous whether default is returned instead of None only or instead of both None and Uninhabited

[randolf-scholz]

To be honest, I do not think this part of the docstring is really accurate to begin with. Looking at the code on the main branch

https://github.com/python/mypy/blob/c6b40df63ce0fecab6bced800ce778d99587c9b8/mypy/checker.py#L7990-
L8045

How about something like:

Returns a 2-tuple:
    The first element is the proposed type, if the expression can be the proposed type.
        (or default, if default is set and the expression is a subtype of the proposed type).
    The second element is the type it would hold if it was not the proposed type, if any.
        (or default, if default is set and the expression is not a subtype of the proposed type).

    UninhabitedType means unreachable.
    None means no new information can be inferred.

[sterliakov]

Also, does this fix #19470 completely or partially? Please update the description to either remove "partially" or remove "fixes" magic word, so that half-resolved issue does not get autoclosed

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

mitmproxy (https://github.com/mitmproxy/mitmproxy)
+ mitmproxy/http.py:607: error: Unused "type: ignore" comment  [unused-ignore]