Stop reloading the middleware on hot updates that match no modules

[matthewp]

Changes

• The middleware hotUpdate handler now ignores updates that Vite matched no modules for, instead of invalidating the middleware on every hot update. Writes outside the module graph — e.g. @astrojs/cloudflare's .wrangler/state files, which are written on every request — previously forced a middleware reload per write, and an SSR re-bundle on Cloudflare, which could cost ~30s per page load.
• Updates that matched modules (the middleware itself, its transitive imports, or any other module in the graph) still invalidate as before, preserving the middleware HMR behavior added in Fix middleware hmr behaviour #17605. Only updates that can't affect the middleware are discarded.

Testing

• Adds a middleware HMR hotUpdate handler unit suite to test/units/middleware/middleware-hmr.test.ts with a fake environment covering the handler's branches: invalidates when the update matched modules, invalidates for unrelated in-graph changes, does not invalidate when no modules matched (the regression), and is a no-op outside server environments.
• The no-modules case fails without the guard.

Docs

• No docs update needed; this restores dev-server behavior that regressed in 7.2.1, with no user-facing API change.

Fixes #17933

[changeset-bot]

🦋 Changeset detected

Latest commit: 9ac9c8d

The changes in this PR will be included in the next version bump.

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[codspeed]

Merging this PR will not alter performance

✅ 21 untouched benchmarks

---

_{Comparing fix/17933-middleware-hmr-rebuild (9ac9c8d) with main (e294953)}