test: add gated integration test suite

[nickvanw]

Summary

• rename the live database harness into a permanent build-tagged integration test suite
• add make test-integration and make test-integration-stress
• document credential setup, table cleanup, and stress knobs in doc/integration-tests.md
• cover real MySQL DDL/DML, schema discovery, Connect/VStream reads, Sync.Handle, schema-aware serialization, checkpoint resumes, idle syncs, repeated mutation bursts, and opt-in burst-load stress

Validation

• env GOCACHE=/tmp/fivetran-go-build-cache go test -tags=integration ./cmd/internal/server/handlers -run '^$'
• env GOCACHE=/tmp/fivetran-go-build-cache go test ./cmd/internal/server/handlers
• env GOCACHE=/tmp/fivetran-go-build-cache go test ./...
• combined live-check worktree with PRs Fix BIT value conversion #83, Parse enum and set values with Vitess #84, Validate SET bitmask conversion #85, Validate row field counts before serialization #86, and Return non-timeout read errors #87 merged: env GOCACHE=/tmp/fivetran-go-build-cache go test ./...
• combined live-check worktree with the real PlanetScale test database: go test -tags=integration ./cmd/internal/server/handlers -run TestIntegration -count=1 -v passed in 168.860s
• combined live-check worktree stress run with the real PlanetScale test database: INTEGRATION_STRESS=1 go test -tags=integration ./cmd/internal/server/handlers -run TestIntegrationStressBurstLoad -count=1 -v passed in 96.738s

Notes

This intentionally does not include database credentials. Real runs require PS_INTEGRATION_CONFIG or DATABASE_* environment variables.