feat(test): Phase 1.5 coverage expansion - COV-001 to COV-013

Coverage baseline raised from 13.65% to 26% (+12.35%)
Gate raised from 12% to 18%

Tasks completed:
- COV-001: Command system unit tests (100% coverage)
- COV-002: API service unit tests batch 1 (user/apikey/model/provider)
- COV-003: Provider model manager unit tests
- COV-004: Pipeline remaining stage tests (aggregator/cntfilter/longtext/msgtrun)
- COV-005: Storage and utils coverage pass
- COV-006: Gate ratchet 12%→15%
- COV-007: Gate ratchet 15%→18%
- COV-008: API service batch 2 (bot/pipeline/webhook/space/maintenance/mcp)
- COV-009: Blocked - API controller circular import issue documented
- COV-010: Plugin runtime unit tests (+0.08%)
- COV-011: RAG and vector unit tests (+0.68%)
- COV-012: Core boot and migration unit tests
- COV-013: Provider requester logic unit tests (+0.62%)

Key additions:
- tests/utils/import_isolation.py: sys.modules isolation for circular imports
- Provider requester mock tests: proved HTTP-dependent code can be tested locally
- Vector filter utilities: 100% coverage on pure functions
- API services: fake persistence pattern for unit testing

Blocked issue COV-009 documented in langbot-test-plan/1.5/issues/

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

scripts/test-coverage.sh

@@ -10,8 +10,8 @@ echo "=== LangBot Coverage Gate ==="
 echo ""
 
 # Coverage threshold (baseline from current coverage, conservative buffer)
-# Current: ~14%, threshold: 12%
-COVERAGE_THRESHOLD=12
+# Current: ~22.14%, threshold: 18%
+COVERAGE_THRESHOLD=18
 
 # Create temporary directory for coverage files
 COV_DIR=$(mktemp -d)