feat(mcp): preserve typed results and policy boundaries

[ZhiXiao-Lin]

Summary

• preserve standard MCP tool metadata: title, output schema, annotations, icons, and _meta
• project structuredContent, protocol metadata, images, and embedded resources without lossy string conversion
• materialize bounded content-addressed artifacts with race-safe digest verification
• make tool-owned confirmation metadata escalation-only
• let explicitly scoped workers see parent-hidden tools while retaining the stricter parent and worker execution decisions

Validation

• 178 MCP-focused Core tests
• 17 worker and delegated-permission tests
• explicit parent-deny and HITL-escalation regressions
• cargo clippy -p a3s-code-core --lib -- -D warnings
• all-target Clippy with only the pre-existing upstream agent_api/tests.rs err_expect lint excluded
• cargo fmt --all
• git diff --check