Risky behavior now covered

• Plugin catalog staleness / partial marketplace: local or failed remote refreshes must mark availablePluginsPartial so clients keep cached marketplace rows; successful remote refreshes must not.
• Epoch invalidation: in-flight remote catalog emits with a stale onlyIfEpoch are dropped; invalidate-during-refresh queues a follow-up refresh at the new epoch.
• Remote refresh failures: build errors surface as internal_error and never emit a clearing plugins_catalog.
• Plugin detail fail-closed: workspace-scoped misses skip remote lookup; ambiguous/missing ids emit validation_failed; remote fallback only when local catalog misses without a workspace scope.
• Provider auth / model selection gates: blank model id, unsupported provider, busy setModel, blank/unknown auth method ids, OpenCode-only API-key copy sibling gate, and persist-after-apply failure reporting.

Test files added/updated

• test/session/pluginCatalogService.test.ts (new)
• test/session/providerAuthManager.test.ts (new)
• src/server/session/PluginCatalogService.ts — tiny optional-deps seam (default wiring unchanged) so epoch/error paths are injectable, matching the MCP validation flow pattern

Why these tests materially reduce regression risk

These two extracted session collaborators sit on high-blast-radius paths (plugin marketplace UX and provider credential/model changes) and previously lacked dedicated fail-closed unit coverage. Prior coverage runs claimed the gaps; main has been unchanged at e2d5f95e, so this run fills them without duplicating open PRs #331–#346.

Validation

• Targeted: pluginCatalogService, providerAuthManager, platform-boundary — 24 pass
• bun run typecheck — clean
• Biome on touched files — clean