Implement direct KRM types and generate.sh for ComputeBackendService (#10049)

Configure and run generate-types and generate-mapper for
ComputeBackendService under apis/compute/v1beta1/generate.sh. Implement
strict schema-compatible KRM types in backendservice_types.go and custom
mapper functions in mappers.go. Ensure 100% equivalence in the generated
CRD schema.

Fixes #9960

Author: justinsb

.gemini/skills/crd-mapper-fuzzer-existing-type/journal/computebackendservice.md

@@ -0,0 +1,17 @@
+# ComputeBackendService Transition Journal
+
+During the transition of `ComputeBackendService` to direct KRM Go types, several crucial lessons and patterns were identified:
+
+1. **Casing Matching in Generator:**
+   The `generate-mapper` tool expects specific casing suffixes for custom structs (e.g., `Cdn` instead of `CDN`, `Iap` instead of `IAP`, `Http` instead of `HTTP`).
+   By naming the handcoded structs and their mapping functions exactly with the correct suffix (e.g., `BackendServiceCdnPolicy_v1beta1_ToProto` instead of `BackendServiceCDNPolicy_v1beta1_ToProto`), the generator successfully skipped generating conflicting mapper functions in `mapper.generated.go`.
+
+2. **Isolated Secret Reference Types:**
+   Because some common structs like `SecretKeyRef` can have subtle import differences or confuse `controller-gen`, defining a local `BackendServiceSecretKeyRef` under `backendservice_types.go` isolates the schema definitions and ensures successful build and clean, identical JSON schema generation.
+
+3. **Flat Mapping and Regional/Global Logic:**
+   The `connectionDrainingTimeoutSec` is flat-mapped in KRM but lives inside a nested `connection_draining` property in the proto. Custom logic in the top-level `ToProto`/`FromProto` mappers cleanly maps this flat field into the nested proto structure.
+   Additionally, regional vs global mapping is handled dynamically by parsing the last component of the region URL or checking if the region is global.
+
+4. **Strict Schema Equivalence:**
+   Using `dev/tasks/diff-crds` frequently allowed ensuring that we kept the schema absolutely identical to the baseline CRD, including retaining the correct `tf2crd=true` label, metadata labels, and required field lists in nested structures.

apis/compute/v1beta1/computebackendservice_types.go

@@ -25,6 +25,7 @@ cd ${REPO_ROOT}/dev/tools/controllerbuilder
 go run . generate-types \
   --service google.cloud.compute.v1 \
   --api-version compute.cnrm.cloud.google.com/v1beta1 \
+  --resource ComputeBackendService:BackendService \
   --resource ComputeBackendBucket:BackendBucket \
   --resource ComputeExternalVPNGateway:ExternalVpnGateway \
   --resource ComputeFirewall:Firewall \