fix(integration): green gate with tracked streaming skip, gemini coverage, robust probe

Addresses coordinator review of the Task 15 integration suite: 1 critical
ruling, 5 important, 3 folded. make integration now exits 0.

- integration_test.go: TestStreamingIncrementalityProbe restructured —
  content assertion (5 deltas + [DONE], in order) always runs; the raw-TCP
  timing assertion (unchanged, >=3 gaps >=100ms) is gated behind
  INTEGRATION_STREAMING=1 with t.Skipf naming traefik/traefik#10269,
  verified to still genuinely fail when re-armed (not silently disabled).
- Makefile: integration-up/integration-wait moved from prerequisites into
  the integration recipe body as go test ./... -count=1
ok  	github.com/lukaszraczylo/traefik-llmgateway	6.065s sub-invocations with per-stage
  status capture, so integration-down always runs even when bring-up or
  the health wait fails; verified with a synthetic failure. Added
  .NOTPARALLEL (the integration targets are stateful).
- translate_gemini.go: fixed the same yaegi multi-value-assignment panic
  shape as the prior sse.go fix, on the live max_tokens path. Added a
  gemini chat completion assertion to integration_test.go (renamed
  TestUnifiedChatAllProviders) so this path now runs under real Yaegi.
- providers.go, translate_anthropic.go: documented the never-wrap
  contract on providerHTTPError/translateError (routes_unified.go asserts
  their concrete type directly; errors.As panics under yaegi on these).
- integration_test.go: TestCrossReplicaLimits now FLUSHALLs Redis first,
  for idempotence under integration-keep.
- mock/main.go: probe's header-terminator search now accumulates a
  rolling buffer across reads instead of only checking the latest one.
- Folded: hot-reload deadline 10s -> 20s; dynamic.yml's real-upstream
  baseUrl is now a template (dynamic.yml.tmpl) rendered via
  INTEGRATION_REAL_BASEURL (default https://llmgw.example.com), not a
  hardcoded-only hostname.

See task-15-report.md's addendum for full verification detail.

translate_gemini.go

@@ -363,13 +363,23 @@ func geminiRequestFromOpenAI(req map[string]any) (map[string]any, error) {
 	genConfig := map[string]any{}
 	var maxTokens int64
 	var hasMaxTokens bool
+	// maxTokens and hasMaxTokens are set as two separate statements, not one
+	// "maxTokens, hasMaxTokens = n, true" multi-value assignment: yaegi
+	// v0.16.1 panics interpreting a multi-value assignment mixing a
+	// variable and a literal in one statement on this exact live path (the
+	// same class of bug as sse.go's readSSE dispatch and registry.go's
+	// resolveAgainst — see those files' comments). Verified empirically
+	// under real Traefik (Task 15's integration suite, the gemini chat
+	// completion path).
 	if v, ok := req["max_completion_tokens"]; ok {
 		if n, ok2 := toInt64(v); ok2 {
-			maxTokens, hasMaxTokens = n, true
+			maxTokens = n
+			hasMaxTokens = true
 		}
 	} else if v, ok := req["max_tokens"]; ok {
 		if n, ok2 := toInt64(v); ok2 {
-			maxTokens, hasMaxTokens = n, true
+			maxTokens = n
+			hasMaxTokens = true
 		}
 	}
 	if hasMaxTokens {