Addon-Vitest: Subscribe for run completion before triggering it

[tsushanth]

Fixes #35289

The TRIGGER_TEST_RUN_REQUEST handler in preset.ts sends TRIGGER_RUN to the store and then subscribes to completion events. If the run finishes quickly — which happens for focused or small story sets via @storybook/addon-mcp — the TEST_RUN_COMPLETED event can fire before the subscription is in place. The response channel never emits TRIGGER_TEST_RUN_RESPONSE, and the MCP caller hangs until it times out.

The fix registers the store.subscribe call before store.send, so any completion event that fires synchronously or near-synchronously is captured. The logic inside the subscriber is unchanged.

Summary by CodeRabbit

• Bug Fixes
  • Fixed a timing issue in programmatic test runs where very fast completions could be missed.
  • Improved reliability by ensuring the test-run listener is set up before dispatching the trigger, so completion, error, and cancellation outcomes are captured consistently.

Manual testing

See instructions on #35289. Use canary 0.0.0-pr-35291-sha-a08be029

[coderabbitai]

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Package Benchmarks

^{Commit: a08be02, ran on 26 June 2026 at 13:59:48 UTC}

The following packages have significant changes to their size or dependencies:

storybook

	Before	After	Difference
Dependency count	72	72	0
Self size	21.88 MB	21.68 MB	🎉 -201 KB 🎉
Dependency size	36.44 MB	36.44 MB	0 B
Bundle Size Analyzer	Link	Link

@storybook/vue3-vite

	Before	After	Difference
Dependency count	102	109	🚨 +7 🚨
Self size	36 KB	36 KB	🎉 -24 B 🎉
Dependency size	42.99 MB	44.15 MB	🚨 +1.16 MB 🚨
Bundle Size Analyzer	Link	Link

@storybook/cli

	Before	After	Difference
Dependency count	204	204	0
Self size	821 KB	821 KB	🚨 +1 B 🚨
Dependency size	91.26 MB	91.06 MB	🎉 -201 KB 🎉
Bundle Size Analyzer	Link	Link

@storybook/codemod

	Before	After	Difference
Dependency count	197	197	0
Self size	32 KB	32 KB	0 B
Dependency size	89.74 MB	89.54 MB	🎉 -201 KB 🎉
Bundle Size Analyzer	Link	Link

create-storybook

	Before	After	Difference
Dependency count	73	73	0
Self size	1.09 MB	1.09 MB	🎉 -66 B 🎉
Dependency size	58.32 MB	58.12 MB	🎉 -201 KB 🎉
Bundle Size Analyzer	node	node