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.

.traefik.yml

@@ -1,6 +1,17 @@
 displayName: LLM Gateway
 type: middleware
 import: github.com/lukaszraczylo/traefik-llmgateway
+# basePkg: Traefik's Yaegi middleware loader (pkg/plugins/middlewareyaegi.go,
+# newYaegiMiddlewareBuilder) binds the interpreted package under
+# strings.ReplaceAll(path.Base(import), "-", "_") — "traefik_llmgateway" —
+# unless this field overrides it. The package itself declares
+# `package traefikllmgateway` (no underscore), so without this override a
+# real Traefik instance fails plugin load with
+# `failed to eval New: ...: undefined: traefik_llmgateway`, discovered
+# running the plugin in real Traefik v3.5 for the first time (Task 15's
+# integration suite). Required for both Traefik itself and the Plugin
+# Catalog analyzer (piceus), which uses this identical mechanism.
+basePkg: traefikllmgateway
 summary: Multi-provider LLM gateway with groups, per-user keys, token/cost limits, MCP/A2A proxy.
 testData:
   providers: