A platform change now runs the plugin suites before it merges

The plugin repository compiles its code against the platform. When the platform removes something the plugins still use — a type, and since yesterday also a single field or method of a type that stays — the plugins stop building. Until now the platform had no way to see that: its own checks do not build the plugins, and the plugins' checks build against the platform's main branch, so the failure appeared there, later, on pull requests that had nothing to do with it. On 2 September a two-line removal did exactly this: every plugin pull request was red for three hours, and the job that went red reported that nothing was tested — an entire shard's suites silently stopped running for everyone.

A platform pull request that changes public code now runs the plugin suites against itself. The plugin repository is asked to build its code and run its portal-host tests against the pull request's merge result, and the outcome comes back as one check on the platform pull request — green only when the plugins passed against exactly that change. A pull request that changes no public code says so and runs nothing. A pull request the plugins cannot answer does not pass quietly; it fails and says that nobody answered.

Two smaller things landed with it. The check that asks a removing pull request to name its counterpart now notices a removed member as well as a removed type. And a waiver that rests on a search of the live mesh must quote that the search actually ran — a search that answered "nothing was searched" had been read as "nothing found".