fix(ui_bundler): revalidate asset-bundle cache signature so patched webui assets invalidate without restart

[wgnrai]

Problem

get_ui_asset_bundle() cached the UI asset bundle in an in-memory cache keyed by entry-URL hash only, and returned the cached bundle WITHOUT recomputing _bundle_signature() (which already covers plugin webui file mtimes). Any change to core or plugin webui assets after process start (e.g. a plugin patching _model_config/webui/model-config-store.js) left the server serving a stale bundle version via /ui/asset-bundle until restart. The stale version kept splash.html's service-worker registration unchanged, so browsers kept executing pre-patch JS — in our case a preset-save loop that silently dropped a model slot on every save. We hit this on a live deployment with a plugin-patched preset UI.

Root cause

The cache hit path never revalidated against the signature. The signature function already existed and already covered all asset roots; the bug was purely that it was not checked on cache hits.

Fix

Recompute the signature on every call (stat-pass over bundle roots, ~300 files, once per page load — negligible); reuse the cached bundle only on signature match; rebuild + re-cache on mismatch. Clients self-heal automatically: new bundle version → splash registers the service worker under the new URL → activate handler purges old caches.

Verification

• Regression test test_bundle_cache_invalidates_when_plugin_asset_changes (touch plugin asset → next get_ui_asset_bundle returns new version + new content)
• 4/4 ui_bundler tests + 18/18 related webui asset suites pass
• End-to-end confirmed on a live deployment (patched preset-save UI now persists after the fix; it failed 5× before)

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[wgnrai]

Refreshed this against latest main. The bundle cache still serves stale patched assets after a webui change without the signature revalidation, so the fix still applies.