## Why
PR #27388 lets models read child resources referenced by backend plugin
skills without an executor. The integration fixture should prove that
real flow: the injected `SKILL.md` advertises a child `skill://`
resource, and `skills.read` resolves that exact resource through the
backend provider.
This is stacked on #27388.
## What changed
- Adds a child-resource link to the backend skill fixture and asserts
that it reaches model context.
- Tightens the end-to-end skills test around `skills.list` followed by
`skills.read` for the referenced resource.
- Splits the existing app-server `mcpResource/read` coverage into a
focused test so the generic RPC path remains covered independently.
## Validation
- Adds app-server integration coverage for both the referenced backend
skill resource and the generic MCP resource read path.
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