fix: preserve Mirror context attributes in build_child_ctx

[vasspilka]

Summary

Mirror.build_child_ctx/2 was rebuilding the child Context from a hard-coded subset of fields (service_opts, resource, attachment, actor, tenant), silently dropping anything else the parent context carried — e.g. expected_md5 and any future Context additions.

Replace the rebuild with %{parent | service_opts: child.opts} so every attribute on the parent propagates to the child, with only the per-service opts overridden.

Tests

New context propagation describe block in test/ash_storage/service/mirror_test.exs covers:

• download fails with :checksum_mismatch when expected_md5 is wrong (proves the field now reaches the child)
• download succeeds when expected_md5 matches
• fall-through to the secondary service still propagates expected_md5

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

🤖 Generated with Claude Code

[zachdaniel]

🚀 Thank you for your contribution! 🚀