feat: add init/1 callback to Ash.Policy.Check

[nallwhy]

Contributor checklist

Leave anything that you believe does not apply unchecked.

• I accept the AI Policy, or AI was not used in the creation of this PR.
• Bug fixes include regression tests
• Chores
• Documentation changes
• Features include unit/acceptance tests
• Refactoring
• Update dependencies

Summary

Closes #1118

[Copilot]

Pull request overview

This PR introduces an init/1 callback for Ash.Policy.Check implementations so policy check options can be validated/normalized at compile time when policies are defined.

Changes:

• Adds init/1 as a new callback on Ash.Policy.Check and calls it during DSL transform.
• Provides default init/1 implementations for Ash.Policy.SimpleCheck, Ash.Policy.FilterCheck, and manual checks (use Ash.Policy.Check).
• Adds tests covering successful option normalization and compile-time failure when init/1 returns an error.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
lib/ash/policy/check.ex	Adds the init/1 callback and invokes it during policy transform, plus defaults for manual checks.
lib/ash/policy/simple_check.ex	Adds a default init/1 and makes it overridable for simple checks.
lib/ash/policy/filter_check.ex	Adds a default init/1 and makes it overridable for filter checks.
test/policy/simple_test.exs	Adds coverage for init/1 normalization and error propagation during compilation.

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[zachdaniel]

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