Fix flaky bundle verifier: stop parsing partial stdout lines

[HelloThisWorld]

Summary

The post-merge main run flaked on macOS/node 20 in "Verify the isolated plugin bundle" (failing job) with an uncaught non-JSON on stdout error right after a successful MCP handshake. The bundle is fine — the verifier script has a latent parsing race that v1.2 made much more likely to fire.

waitFor polls the accumulated stdout buffer every 50 ms and ran JSON.parse over every split('\n') segment — including the trailing partial line while a response is still streaming. tools/list grew to 50 tools in v1.2, a payload of hundreds of KB that always arrives across several pipe chunks; a poll landing between two data events saw an incomplete JSON fragment and rejected it as a stdout-purity violation. That is why the identical step passed on the PR run minutes earlier and on three of four platforms in the failing run: it is a timing race, and the larger payload widened the window.

The fix parses only complete lines (everything up to the last newline) and leaves an in-flight fragment for the next poll. A COMPLETE non-JSON line still fails hard, so the self-containment guarantee the check exists for is unchanged. pnpm verify:plugin-bundle passes locally (11/11).

Checklist

• pnpm lint, pnpm typecheck, and pnpm test pass locally — script-only change; the verifier itself (pnpm verify:plugin-bundle) passes 11/11, and the full suites were green on this exact tree at the v1.2 merge
• pnpm check:public-contracts passes — no contract touched
• Documentation updated for any user-visible behavior change — none (CI script only)
• CHANGELOG entry added for any user-visible change — none (no contract snapshot changed)
• Everything is in English (code, comments, docs, commit messages)
• No employer or client proprietary content — examples and fixtures are synthetic
• No credentials, tokens, or secret values anywhere in the diff, fixtures, or recorded test output