test(startup): build the host composition in CI, not only when somebody runs it

[BechsteinDigital]

Found while checking whether the three merged PRs (#385, #386, #387) actually hold up outside the test hosts.

The gap

src/Host/Dev/Program.cs carries this claim:

if someone breaks AddCalloraHost or the order these have to run in, this stops building rather than the next distribution finding out

That held for compile errors only. A missing DI registration compiles cleanly and fails at builder.Build() — and nothing in CI ever made that call. AddCalloraHost had exactly one caller in the repository, and it runs when a person presses F5.

So a service registered by hand in a test host and absent from the real composition came through the suite green and would have taken down the dev stack at startup. That is precisely the shape of the last three PRs: WorkspaceReach, PluginSelfService and ITenantPluginDelegationStore are registered manually in every test host that uses them.

What the gate covers

AddCalloraHost sets ValidateScopes and ValidateOnBuild, so the build itself says something: every registered dependency is checked for resolvability and for a scoped service captured by a singleton.

And that is where it stops — measured, not assumed. Remove PluginSelfService from the composition and the host still builds: a service registered nowhere is not validated either, and a controller's [FromServices] parameter resolves only when it is called. Only the second test — the one naming the tenant-level services — goes red.

I had written the opposite in the test comment first ("covers everything that comes later, nobody has to extend it"). The experiment showed it was wrong, and the comment now says what actually holds: whatever is consumed only through [FromServices] has to be listed explicitly or nothing covers it.

Also

The Program.cs comment is corrected in the same move and points at the test, so the two stay in step — the test makes the same calls in the same order, and a composition that drifts from it would be testing something nobody ships.

Callora.Core.Tests      1841 passed, 0 failed, 0 skipped
Callora.Analyzers.Tests   34 passed, 0 failed, 0 skipped