feat: add a finalizer option to VerifyTestMain

[sb123sb123]

Summary

Add goleak.WithFinalizer for VerifyTestMain.

The callback runs after m.Run() and before goroutine leak detection, so package-level providers or other resources can be finalized before the final scan. Passing the option to Find is rejected, matching the existing option validation pattern.

Fixes #133

Validation

• go test ./... -run "^(TestVerifyTestMain|TestFind)$" -count=1
• go test ./... -count=1
• go vet ./...
• git diff --check
• A focused -race run was attempted but the Windows environment lacks a usable C compiler and headers (errno.h, stdlib.h, and windows.h).

AI disclosure

This change was prepared with assistance from an AI coding agent. The issue state, open PRs, repository policy files, diff, and test results were checked on the target branch.

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