fix(grpc): allow omitted push config ids

[56wj]

fix(grpc): support omitted push notification config ids

Summary

Follow up on #1092: GetTaskPushNotificationConfigParams permits an omitted configuration ID, and DefaultRequestHandler resolves null/empty IDs to the task ID. However, GrpcTransport still rejects an omitted ID before sending the RPC, unlike RestTransport.

• Remove the stale gRPC-only non-null assertion on the optional configuration ID.
• Encode a null configuration ID as the protobuf default empty string; preserve explicit IDs and tenant resolution.
• Add five in-process gRPC tests for constructor/builder omission, empty ID, explicit ID, and tenant precedence.
• Add REST tests asserting that null/empty IDs use the default-configuration GET URL rather than LIST.
• Add grpc-inprocess only as a test-scoped dependency. No production dependencies or public API changes.

Verification

Before the fix, GrpcTransportTest ran 5 tests with 3 errors (IllegalArgumentException: Parameter 'id' may not be null), all on omitted-ID paths.

After the fix:

mvn -pl client/transport/grpc,client/transport/rest -am test

557 tests across the two transports and their dependency modules passed, with 0 failures, errors, or skipped tests. This is the selected reactor, not the entire repository.

• Follow the CONTRIBUTING guide
• Conventional commit title
• Tests pass
• No README change needed: this restores the existing documented optional-ID contract

Refs #1012; follow-up to #1092. The original issue's null-ID premise predates #1092; this PR addresses the remaining client-side omission rather than choosing a new blank-ID policy.

Developed with AI coding assistance, with failing-before/passing-after regression evidence.

[kabir]

Thank you @56wj