Fix partial state after hookspec validation and historic replay failures(#761)

[starx710]

Summary

Make failed hookspec validation and historic plugin registration recoverable.

Problem

add_hookspecs() installs specifications before validating existing implementations. A validation error leaves partially installed specifications and prevents a retry, even after the invalid plugin is removed.

When historic replay raises during register(), the plugin and implementations processed earlier remain registered. Replay may also add specifications or register other plugins, so restoring the entire manager would discard valid user changes.

Solution

• Validate every candidate specification using a temporary HookCaller, then install the prepared specification and historic state. Keep existing caller identities and implementations.
• On registration failure, remove only the implementation objects added by that registration. Remove its newly created callers only if they remain unused and have no specification.
• Track active registration ownership so cleanup preserves replacement or nested registrations, including re-registration of the same object without hook implementations.
• Catch BaseException for cleanup and use a bare raise to preserve the original exception and traceback. Avoid plugin equality during rollback and retain replay side effects.
• Update the existing 761.bugfix.rst fragment.

Regression Tests

Add 20 test cases (after parametrization) and strengthen the existing late-spec validation test. Coverage includes:

• Invalid implementations, retry, multi-spec failures, caller/subset identity, duplicate specs and warnings treated as errors.
• Implementation and result-callback replay failures, including RuntimeError and KeyboardInterrupt, with exception identity and traceback checks.
• Cleanup of earlier implementations and unused new callers; preservation of existing callers, implementations, specs and history; successful retry.
• Specs and other plugins added during replay, adopted callers, replacement/re-registered/blocked/removed registrations, and plugin equality that raises.

Validation

Tested on Windows with CPython 3.12.13:

• uv run pytest -q --tb=short: 239 passed.
• uv run pre-commit run --all-files: all hooks passed, including Ruff, formatting and mypy.
• git diff origin/main --check: passed.
• Running the final regression checks against an isolated, unmodified upstream source snapshot (6995073) produced 19 failures and 2 passing controls. The working checkout was unchanged.
• Coverage run: _manager.py has no uncovered statements; combined statement/branch coverage is 99%.

Other Python versions and platforms await CI after PR creation.

Related Issue

Fixes #761.

Related: #779 also addresses historic registration rollback.

Prepared with OpenAI Codex; local tests and self-review were performed before submission.