fix: propagate shared context to nested forms in add_form and validate

[nallwhy]

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• Bug fixes include regression tests
• Chores
• Documentation changes
• Features include unit/acceptance tests
• Refactoring
• Update dependencies

The previous fix (5db38dc) only threaded shared context through handle_forms. Nested forms created by add_form/3 and re-created during validate_nested_forms still called for_action without :context, so :shared context was lost whenever a form was modified after creation.

Propagate shared context in both code paths, and extract the Map.take(context, [:shared]) filter into a shared_context/1 helper used across handle_forms, validate_nested_forms, and do_add_form.

[Copilot]

Pull request overview

This PR fixes nested form context propagation so that only the parent form’s :shared context is preserved when nested forms are created or recreated through initial params, add_form/3, and validation.

Changes:

• Adds a shared_context/1 helper to centralize filtering context down to :shared.
• Threads shared context through validate_nested_forms/6 and do_add_form/6 nested for_action calls.
• Adds regression tests for initial params, add_form, and validate-created nested forms.

Reviewed changes

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

File	Description
lib/ash_phoenix/form/form.ex	Propagates filtered shared context to nested forms across form handling, validation, and add-form paths.
test/form_test.exs	Adds/renames tests verifying only :shared context reaches nested forms.

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[zachdaniel]

🚀 Thank you for your contribution! 🚀