Fix startup and per-turn provider registry hot paths by keeping primary-model startup discovery on metadata-only provider entries and by keeping capability provider fallback loads scoped to manifest-derived owners, including explicit empty scopes when no bundled owner exists.
Evidence:
- Reproduces the reported code paths from #73729, #73835, and #73793: startup prewarm was able to enter provider/model discovery that loaded plugin runtime, and capability lookups could bypass active registry reuse or broaden fallback registry loads.
- Fix threads providerDiscoveryEntriesOnly through models-config planning into plugin discovery.
- Fix reuses active non-memory/non-speech capability providers even with explicit plugins.entries.
- Fix keeps fallback registry loads scoped with onlyPluginIds, including [] for no-owner media capability checks.
- Local targeted tests passed for gateway startup, models config, provider discovery, capability providers, and web provider runtimes.
- Testbox pnpm check:changed passed.
- Testbox pnpm build passed.
- GitHub CI required checks passed on e5e6fe1d52.
Fixes#73729.
Fixes#73835.
Fixes#73793.
Supersedes #73794.
