test: cover referenced backend skill reads without an executor

[jif-oai]

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.