[Test] Add E2E coverage for the language signal

[yu3zhang1]

Related #3178

Purpose

Add e2e test for the language routing signal (config.SignalTypeLanguage).
It currently has only Go unit tests for its classifier logic
(language_classifier_test.go) and no end-to-end behavioral coverage
through a deployed request path, as required by #3178.

This PR adds:

• An es (Spanish) language rule and a spanish_language decision
  (priority 25) to e2e/profiles/ai-gateway/values.yaml.
• e2e/testcases/language_routing.go, following the existing
  pkgtestcases.Register(...) pattern used by event_routing.go,
  asserting the x-vsr-selected-decision and x-vsr-matched-language
  response headers.
• e2e/testcases/testdata/language_routing_cases.json with two positive
  cases (distinct Spanish-language queries) and one negative case (an
  English, non-Spanish query).
• language-routing registered in BaselineRouterContract, a new row in
  signal_routing_contract_test.go, and the ENVOY_AI_GATEWAY_CI_TESTS
  list in .github/workflows/integration-test-k8s.yml — all three
  registration points needed for the test to actually run in CI, added
  up front based on review feedback from [Test] Add E2E coverage for the event signal #3399.

Owning Workgroup: Evaluation & Quality (wg/evaluation-quality).

Test Plan

• gofmt -l e2e/testcases/language_routing.go
• go vet ./... (run from e2e/)
• go build ./... (run from e2e/)
• golangci-lint run ./testcases/... ./pkg/testmatrix/... (run from e2e/)
• go test ./testcases/... -run TestProfilesSelectSignalRoutingContracts
• make e2e-test-specific E2E_TESTS="language-routing"

Test Result

• gofmt / go vet / go build: pass.
• golangci-lint: 0 issues in the files this PR touches (pre-existing
  issues in unrelated files under e2e/testcases/ were left untouched).
• TestProfilesSelectSignalRoutingContracts: pass, confirms the
  envoy-ai-gateway profile selects language-routing.
• make e2e-test-specific: not run against a live cluster locally
  (same local Kubernetes/network constraints noted on [Test] Add E2E coverage for the event signal #3399). Deferring
  final e2e confirmation to CI on this PR.
• Manually verified the priority ordering in values.yaml: the new
  spanish_language decision (priority 25) sits between thinking_decision
  (20) and urgent_request (30), and its Spanish-language test queries
  don't contain any of the other decisions' trigger keywords, so the new
  test cases can't be shadowed by an unrelated decision.

---

AI assistance (Claude Code) was used to draft this change, under my review;
I read and understand the diff and take responsibility for it.

🤖 Generated with Claude Code

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[yu3zhang1]

E2E (envoy-ai-gateway) runs successfully:

LANGUAGE ROUTING TEST RESULTS
Total Tests: 3
Correct: 3 (100.00%)
[Test] Language routing test completed: 3/3 correct (100.00% accuracy)

[drivebyer]

Non-blocker: the load/result/print scaffolding here mirrors event_routing.go almost verbatim. Could you extract a shared helper in this PR and reuse it in both cases?

[yu3zhang1]

Non-blocker: the load/result/print scaffolding here mirrors event_routing.go almost verbatim. Could you extract a shared helper in this PR and reuse it in both cases?

Done. Move the shared code code to signal_routing_helpers.go. PTAL, thanks!