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b291fbdc - [ty] Reuse common TypedDict constraints through intersections (#26747)

[ty] Reuse common TypedDict constraints through intersections (#26747) ## Summary Prior to this change, the common-protocol-constraint optimization for unions of `TypedDict`s did not apply after an `isinstance(value, dict)` check. Narrowing represents each surviving arm as an intersection with `Top[dict[Unknown, Unknown]]`, so a call like `dict(value)` fell back to combining equivalent generic protocol constraints independently across every union member: ```python from typing import Literal, TypedDict class A(TypedDict): tag: Literal["a"] class B(TypedDict): tag: Literal["b"] def copy(value: A | B | str) -> None: if isinstance(value, dict): reveal_type(dict(value)) # dict[str, object] ``` With a sufficiently wide union, that fallback exhibited pathological runtime. On the issue's 21-arm reproduction, the pre-fix build did not complete within ten seconds; the optimized path completes in under 0.2 seconds. We now recognize intersections containing a positive `TypedDict` as `TypedDict` alternatives, while retaining the complete intersection when comparing its protocol constraints with `Mapping[str, object]`. Closes https://github.com/astral-sh/ty/issues/3974. Co-authored-by: Nate Bracy <nate@bracy.dev>