[cvas22]

Fixes #13877

Proposed Changes

• The activator now adds every PEM-encoded CA found in ca.crt of the routing-serving-certs secret to its upstream trust pool (via x509.CertPool.AppendCertsFromPEM), instead of only the first PEM block. This allows multiple CAs to be trusted at once, e.g. during CA rotation or with a bring-your-own CA bundle.
• Fixes a nil pointer dereference: if ca.crt contained non-PEM data, pem.Decode returned a nil block and block.Bytes panicked in the informer handler. Now a warning is logged and the CA is ignored, matching the existing handling of invalid trust-bundle ConfigMaps.
• Adds TestReconcile cases "Multiple CAs in secret" and "Invalid CA in secret". Both fail without the fix (the first times out because only one CA ends up in the pool; the second panics at cache.go:146) and pass with it.

Tests run locally (Windows, Go 1.26): gofmt -l, go vet ./pkg/activator/certificate/, go test ./pkg/activator/certificate/ -count=1, go build ./pkg/activator/... ./cmd/activator/. Not run locally: -race, the full unit test suite, linters beyond go vet, and e2e tests; I'm relying on CI for those.

Disclosure: I prepared this change with the help of an AI coding assistant. I reviewed the diff and tests and take responsibility for them.

Release Note

The activator now trusts all CA certificates contained in the `ca.crt` field of the `routing-serving-certs` secret, not just the first one, and no longer panics if that field holds invalid data.

[knative-prow]

Welcome @cvas22! It looks like this is your first PR to knative/serving 🎉

[knative-prow]

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