fix(vite-dev-server): wait for support file and retry dynamic imports to avoid CI race

[mschile]

• Closes

Additional details

• Why: In CI (and sometimes locally), the Cypress iframe can load and run initCypressTests.js before the Vite dev server has finished serving the support file or spec module, leading to "Failed to fetch dynamically imported module" and flaky failures.
• What's affected: @cypress/vite-dev-server startup and the client-side dynamic imports in the test iframe (support file + spec).
• Implementation: (1) Server: After server.listen(), we now wait until the support file URL returns HTTP 200 (waitUntilUrlReady) so "dev server ready" is deterministic. New waitForSupportFile.ts builds the same URL path the client uses and polls until 2xx. (2) Client: Dynamic imports for the support file and spec(s) are wrapped in retryDynamicImport() (up to 4 attempts, 400ms apart) for retryable errors (e.g. "Failed to fetch", "dynamically imported module", "Importing a module script failed").

Steps to test

• Run component tests (or system tests that use the Vite dev server) in an environment where the server can be slow to serve modules (e.g. CI or under load). Before: occasional "Failed to fetch dynamically imported module"; after: server waits for support file and client retries imports.
• New unit tests: yarn workspace @cypress/vite-dev-server test -- waitForSupportFile.spec.ts

How has the user experience changed?

• Before: Intermittent failures when the iframe requested the support/spec module before Vite had it ready.
• After: Dev server is considered ready only when the support file is servable, and the client retries transient import failures, reducing flakiness.

PR Tasks

• Have tests been added/updated?
• [na] Has a PR for user-facing changes been opened in cypress-documentation?
• [na] Have API changes been updated in the type definitions?

[cypress]

cypress    Run #69435

Run Properties:   Passed #69435  •  4efbba5f0a: updates

Project	cypress
Branch Review	mschile/vite_failed_dynamic_import
Run status	Passed #69435
Run duration	08m 48s
Commit	4efbba5f0a: updates
Committer	Matthew Schile
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Test results
Failures	0
Flaky	0
Pending	1
Skipped	0
Passing	739
View all changes introduced in this branch ↗︎

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