fix False negative on extra arguments to paramspec callable #1687

[asukaminato0721]

Summary

Fixes #1687

Now synthesize a constructor-backed __call__ attribute for class objects (including the magic-dunder fast path), so protocol/ParamSpec checks see the actual constructor signature instead of the permissive type.__call__, letting kwargs mismatches surface.

Test Plan

Adds a regression test covering the FastAPI-style middleware helper to lock in the new error behavior.

[Copilot]

Pull request overview

This PR fixes issue #1687 by ensuring that ParamSpec-based protocol checks can detect extra keyword arguments when checking class constructors. The fix synthesizes a __call__ attribute for class objects that exposes the actual constructor signature (derived from __new__/__init__) instead of falling back to the permissive type.__call__.

Key Changes:

• Synthesizes constructor-backed __call__ attribute for class objects during attribute lookup
• Applies to both the magic dunder fast path and regular class object attribute lookup
• Enables proper signature validation for ParamSpec protocols like middleware factories

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
pyrefly/lib/test/paramspec.rs	Adds regression test for FastAPI-style middleware pattern to ensure extra kwargs are detected
pyrefly/lib/alt/attr.rs	Implements synthetic __call__ attribute synthesis using constructor_to_callable for class objects in both magic dunder and regular attribute lookup paths

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[yangdanny97]

Where's the location that looks up __call__ to resolve the param spec? It might be easier to change that to look up the constructor type for classes.

[meta-codesync]

@yangdanny97 has imported this pull request. If you are a Meta employee, you can view this in D88641669.

[ndmitchell]

Review automatically exported from Phabricator review in Meta.

[meta-codesync]

@yangdanny97 merged this pull request in cdab03e.

[yangdanny97]

Thank you!