feat(testing): Support reusable prepared test artifacts

[cameroncooke]

Why

Agents and CI workflows need to prepare tests once and execute them later without falling back to raw xcodebuild. The existing build tools always finished with a normal build action, so they could not expose the reusable products produced by build-for-testing.

What Changed

• Extend the existing simulator, device, and macOS build tools with buildForTesting and optional testProductsPath inputs.
• Return managed .xctestproducts bundles and discovered .xctestrun files as structured build artifacts.
• Let test tools execute either prepared .xctestproducts bundles or advanced .xctestrun inputs without rebuilding, while returning the new .xcresult produced by the run.
• Move conditional next-step selection into tool manifests so normal builds and prepared-test builds advertise the appropriate follow-up without changing existing output behavior.
• Add unit, schema, lifecycle, and MCP/CLI snapshot coverage for successful and failing prepared-test workflows.

Fixes #450

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/xcodebuildmcp@475

commit: c3683ea

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