Tests: centralize contract coverage follow-ups (#48751)

* Plugins: harden global contract coverage

* Channels: tighten global contract coverage

* Channels: centralize inbound contract coverage

* Channels: move inbound contract helpers into core

* Tests: rename local inbound context checks

* Tests: stabilize contract runner profile

* Tests: split scoped contract lanes

* Channels: move inbound dispatch testkit into contracts

* Plugins: share provider contract registry helpers

* Plugins: reuse provider contract registry helpers

CONTRIBUTING.md

@@ -93,6 +93,7 @@ Welcome to the lobster tank! 🦞
   - `pnpm test:extension <extension-name>`
   - `pnpm test:extension --list` to see valid extension ids
   - If you changed shared plugin or channel surfaces, run `pnpm test:contracts`
+  - For targeted shared-surface work, use `pnpm test:contracts:channels` or `pnpm test:contracts:plugins`
   - If you changed broader runtime behavior, still run the relevant wider lanes (`pnpm test:extensions`, `pnpm test:channels`, or `pnpm test`) before asking for review
 - If you have access to Codex, run `codex review --base origin/main` locally before opening or updating your PR. Treat this as the current highest standard of AI review, even if GitHub Codex review also runs.
 - Ensure CI checks pass