Template 19: add a private MCP evidence checklist and clarify internal ACA ingress

Problem

Template 19 documents the individual deployment and test steps, but users troubleshooting private MCP connectivity still need to infer which screenshots and runtime signals together prove the intended private path.

The sample also uses az containerapp create --ingress external inside an internal Container Apps environment. Without an explicit note, external is easy to misread as internet-facing even though the environment boundary remains internal.

Suggested improvement

Add a short "private MCP evidence checklist" to the Template 19 README and testing guide:

1. Foundry configuration: Standard Agent service network injection, delegated agent subnet, and customer-managed/BYO VNet.
2. MCP hosting: Container Apps public network access disabled, VNet subnet configured, and virtual IP internal.
3. Runtime: private DNS answer plus successful initialize, tools/list, tools/call, and Foundry response items mcp_list_tools and mcp_call.

Please also clarify that app-level --ingress external is external to the app within the Container Apps environment boundary; it does not create an internet-reachable endpoint when the environment is internal and public network access is disabled.

Reproduction evidence

I validated the current Template 19 pattern in a clean environment with an internal-only Container Apps MCP server. Private DNS, direct VNet MCP calls, and Foundry agent tool enumeration/invocation all passed.

A sanitized reusable probe and evidence checklist are available here:

https://github.com/doruit/foundry-private-mcp-probe

No private resource identifiers or request/response IDs are included.

Start with the Template 19 README and testing guide, then compare their deployment and test steps with the reported internal-only Container Apps evidence. Done means both documents contain the private MCP evidence checklist and clearly explain that app-level external ingress remains inside an internal environment without public network access.