fix(gateway): correct BackendRef matching and canary header build

[marcoma2018]

Fixes in pkg/trafficrouting/network/gateway/gateway.go:

• getServiceBackendRef: per the Gateway API spec, BackendObjectReference.Kind defaults to "Service" when nil. The previous check ref.Kind != nil && *ref.Kind == "Service" skipped refs without an explicit Kind, so HTTPRoutes emitted by controllers that omit the field (e.g. Envoy Gateway) were never recognised as stable/canary backends and the rollout silently failed to inject canary traffic. Treat nil Kind as Service. Also return (-1, nil) on miss instead of (0, nil) to remove the ambiguity between "not found" and "found at index 0".

• setServiceBackendRef: symmetric fix — refuse only when an explicit Kind other than Service is set, so canary BackendRefs derived from a stable ref with nil Kind can be written back. Replace the manual slice rebuild with an in-place assignment.

• buildCanaryHeaderHttpRoutes: the inner loop indexed matches[k] while k ranged over nonPathMatches. The two slices have different lengths whenever a path-bearing match is not the first element, so headers and query params from the wrong match were grafted onto canary rules. Index nonPathMatches[k] instead.

• buildCanaryHeaderHttpRoutes: the shallow copy canaryRuleMatchBase := *canaryRuleMatch shares the underlying arrays of Headers/QueryParams with the original rule. Subsequent appends could mutate the original HTTPRoute when its slices had spare capacity. Clone the slices before extending; preserve nil-ness to keep output identical to the previous implementation when nothing is appended.

• EnsureRoutes: surface the error from intstr.GetScaledValueFromIntOrPercent rather than silently coercing malformed traffic strings to 0 %.

• EnsureRoutes / Finalise: pass the inbound ctx to client Get/Update inside the retry closure instead of context.TODO(), so cancellation and deadlines propagate.

Tests in pkg/trafficrouting/network/gateway/gateway_test.go:

• Added regression cases covering BackendRefs with nil Kind across the weight, header, and finalise code paths.
• Added a case where matches mix path and non-path entries so that pathMatches and nonPathMatches diverge in length, exercising the index-correctness fix.
• Added a clean-state weight canary case; the existing weight-20 case fed in the final 80/20 shape and only proved idempotence.
• Added TestInitialize, TestEnsureRoutesAndFinalise, and TestEnsureRoutesInvalidTraffic to drive the controller end-to-end against a fake client, covering happy path, idempotence, finalise, rollback, and invalid-input rejection.

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[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.87%. Comparing base (b2600e9) to head (e2ea613).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/trafficrouting/network/gateway/gateway.go	75.00%	1 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #342      +/-   ##
==========================================
+ Coverage   51.38%   51.87%   +0.49%     
==========================================
  Files          66       66              
  Lines        8559     8559              
==========================================
+ Hits         4398     4440      +42     
+ Misses       3575     3530      -45     
- Partials      586      589       +3     

Flag	Coverage Δ
unittests	51.87% <75.00%> (+0.49%)	⬆️

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