fix(node): don't crash TanStack Start apps that use Nitro without a start script (#734)

* fix(node): use the Nitro server entry for TanStack Start apps without a start script

A TanStack Start app built with the nitro Vite plugin outputs
.output/server/index.mjs, not dist/. The srvx fallback pointed at dist/
crashes on startup (ENOENT) and the service 502s. The TanStack
scaffolder's default 'Nitro (agnostic)' deploy option produces exactly
this combination: nitro dependency and plugin, no start script.

Detect the nitro plugin (dependency plus vite config reference) and fall
back to 'node .output/server/index.mjs', matching the Nuxt provider's
handling of the same output layout.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* test: make tanstack-nitro-nostart a full example with an HTTP check

Upgrades the plan-level fixture to a real scaffolder-generated app
(npx @tanstack/cli create --deployment nitro) with a test.json, so the
integration suite builds it and verifies the served page — the crash
this PR fixes now has permanent end-to-end coverage.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>