test: add unit tests for all 10 routers (92 total)
New test files:
- test_conversations.py — list/create/get/update/delete/delete-all, admin enforcement
- test_presets.py — list/create/update/delete, default preset protection
- test_profile.py — get/update/default, length validation
- test_models_router.py — list/ps/show/stats/search-status, connect errors
- test_completions.py — API key auth, FIM passthrough, streaming/blocking, errors
- test_search_route.py — explicit search flow, no results, stream errors
- test_memories.py — edit/search/stats endpoints, validation, admin enforcement
Update AGENTS.md with full test file coverage table and README.md
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