drpcmux: add tests for server interceptors

[shubhamdhama]

This commit adds tests for the server interceptors and RPC handling:

• Tests for unary and streaming server interceptors
• Validation of correct interceptor chaining and execution order
• Error handling and propagation through interceptor chains
• Edge cases with empty interceptor chains

[Copilot]

Pull Request Overview

This PR adds comprehensive test coverage for server interceptors in the drpcmux package. The purpose is to validate the correct behavior of interceptor chaining, execution order, and error handling for both unary and streaming RPC types.

• Added interceptor chaining tests with execution order validation
• Implemented edge case handling for empty interceptor chains and error propagation
• Enhanced HandleRPC functionality to support proper interceptor selection based on RPC type

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
drpcmux/interceptor_test.go	Tests for interceptor chaining, execution order, and error handling with mock implementations
drpcmux/handle_rpc_test.go	Comprehensive tests for HandleRPC method with various interceptor scenarios and RPC types
drpcmux/handle_rpc.go	Refactored HandleRPC method to properly select interceptors based on RPC type and extract message receiving logic

Comments suppressed due to low confidence (3)

drpcmux/handle_rpc_test.go:24

• [nitpick] The field name 't' is not descriptive. Consider renaming it to 'testingT' or 'test' for better clarity.

	t              *testing.T

drpcmux/handle_rpc_test.go:31

• [nitpick] The method name 'mockMethod' is generic and doesn't reflect its actual purpose. Consider renaming to 'handleUnaryRequest' or similar to clarify its role.

func (m *mockRPCImpl) mockMethod(_ context.Context, in *mockMessage) (*mockMessage, error) {

drpcmux/handle_rpc_test.go:40

• [nitpick] The method name 'mockStreamMethod' is generic and doesn't reflect its actual purpose. Consider renaming to 'handleStreamRequest' or similar to clarify its role.

func (m *mockRPCImpl) mockStreamMethod(_ drpc.Stream) error {

[cthumuluru-crdb]

LGTM. Eventually, lets change these tests to use generated protobuf code.