backend: k8cache: percent-encode '+' in cache key fields to avoid delimiter collision

[sid200727]

Summary

This PR fixes a cache-key collision bug by percent-encoding any literal + character in the fields used to build k8cache keys, so the + delimiter can no longer be ambiguous with field content.

Related Issue

Fixes #6123

Changes

-> Updated GenerateKey in backend/pkg/k8cache/cacheStore.go to escape literal + to %2B in apiGroup, kind, namespace, and contextID before joining them with + to form the cache key
-> Added a regression test case to TestGenerateKey in backend/pkg/k8cache/cacheStore_test.go covering a context name containing +

Steps to Test

1. Use a kubeconfig context whose name contains a +, e.g. prod+cluster
2. Call GenerateKey with a request URL and that context name
3. Before this fix: the resulting key has more than 4 +-delimited segments (e.g.apps+deployments+default+prod+cluster), which breaksDeleteKeys's positional namespace-stripping (keyPart[2] = "") andcan cause cache invalidation to target the wrong key
4. After this fix: the + in the context name is escaped to %2B (apps+deployments+default+prod%2Bcluster), so the key always has exactly 4 segments and DeleteKeys parses it correctly
5. Run go test ./pkg/k8cache/... - all existing tests pass unchanged, plus the new regression test

Screenshots (if applicable)

N/A (backend logic fix, no UI changes)

Notes for the Reviewer

-> DeleteKeys itself needs no changes, the fix is fully contained in GenerateKey, since it now guarantees the only + characters in a key are the delimiters
-> No existing test fixtures use + in any field, so this change is fully backward compatible
-> gofmt, go vet, and npm run backend:lint all pass clean

[kubernetes-prow]

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