Restore class-valued hook specifications

[adenzhou1350]

Fixes #794.

PluginCode decorates a class as a hookspec. The static-discovery change in 1.7.0 stopped recognizing that class, so add_hookspecs() raises ValueError.

Restore class-valued specifications without changing hook implementation discovery or evaluating properties/other descriptors. For a class signature, read defaults from its __init__, matching the callable already inspected by varnames(). Classes using object.__init__ have no defaults. Keep the original class in HookSpec.function.

Tests cover module/class/instance namespaces and implicit, required, defaulted, inherited and wrapped constructors. They verify argument metadata, explicit/default hook calls, and that constructors are never executed. A control preserves the existing behavior of ignoring class-valued hook implementations.

Validation

• Final regression tests against unchanged main: 15 failed, 1 control passed.
• Candidate full suite: 235 passed on Windows CPython 3.14 / pytest 9.0.3 and Linux CPython 3.12 / pytest 9.1.1.
• Mypy and cached non-Ruff pre-commit hooks passed; no formatter changes. Local Ruff is blocked by Windows policy and was not rerun elsewhere. The cached pyupgrade hook is 3.21.2 rather than the current 3.22.0, so this is not a full current pre-commit pass.
• Tests used exact source selection with existing dependency environments, not a fresh uv-locked build. Full ScanCode/PluginCode integration was not run.

Reproduction context: #794 (comment)

AI-assisted implementation and validation.

[RonnyPfannschmidt]

this is a prime example of what destroys opensource - unattended ai slop adding a fix that eats review time when the only correct answer is - no this never was intended

the person operating the bot should be ashamed of their behavior