test: integration suite with real Traefik, redis-shared limits, streaming assertions

Adds integration/ (separate module): docker compose stack with real Traefik
v3.5 + localPlugins, mock openai/anthropic/gemini/tool providers, shared
Redis, and a raw-TCP streaming timing probe. 6/7 tests pass; streaming
incrementality fails on a confirmed external yaegi/Traefik bug (see
task-15-report.md), not a plugin defect.

Fixes 4 real Yaegi-interpretation bugs surfaced by running the plugin under
real Traefik for the first time (none caught by go test or yaegi-check,
which never exercises chat/embeddings/streaming/Redis):

- .traefik.yml: add basePkg (Traefik derives the yaegi binding identifier
  from the module path, not the package's own declared name; without this
  override the plugin never loads at all).
- registry.go: resolveAgainst's multi-value return panicked yaegi on an
  inline string concatenation; assign to a local first.
- routes_unified.go: errors.As panicked yaegi on interpreted pointer error
  types; use plain type assertions (both types are always returned bare,
  never wrapped).
- sse.go: readSSE's dispatch closure hit the same multi-value-assignment
  panic as registry.go; split into separate statements.
- resp.go: respErr as a defined string type was indistinguishable from a
  plain string in a yaegi type assertion, misreading every successful
  Redis SELECT reply as failed and silently disabling rate limiting;
  changed to a struct.

integration/mock/Dockerfile

@@ -0,0 +1,15 @@
+# Build context is integration/ (see docker-compose.yml's build.context),
+# so this Dockerfile can reach both go.mod and the mock/ package.
+FROM golang:1.22-alpine AS build
+WORKDIR /src
+ENV CGO_ENABLED=0
+COPY go.mod ./
+COPY mock ./mock
+RUN go build -o /out/mock ./mock
+
+FROM alpine:3.20
+RUN apk add --no-cache wget ca-certificates
+COPY --from=build /out/mock /usr/local/bin/mock
+EXPOSE 8080
+ENTRYPOINT ["/usr/local/bin/mock"]
+CMD ["-mode", "openai"]