docs(openapi): add OpenAPI documentation for greeter services

- Add comprehensive OpenAPI 3.0.3 documentation for HelloGreeterAPI and SecondGreeterAPI
- Include definitions for all API endpoints, request/response models, and error handling
- Generate OpenAPI YAML using protoc-gen-openapi plugin
- Update buf.gen.yaml configuration for OpenAPI generation

buf.gen.yaml

@@ -10,6 +10,7 @@ managed:
   #  disable:
   #    - file_option: go_package_prefix
   #      module: buf.build/bufbuild/protovalidate
+
 plugins:
   # comment with error: duplicate generated file name "openapi.yaml". Generation will continue without error here and drop the second occurrence of this file, but please raise an issue with the maintainer of the plugin.
   - local: protoc-gen-openapi
@@ -19,8 +20,10 @@ plugins:
       #      - depth=2 # Recursion depth of the loop message. The default value is 2
       #      - default_response=false # Adds a default response message. If true, a default response is automatically added for actions using the google.rpc.Status message. This is useful if you use envoy or grpc-gateway for transcoding, as they use this type as the default error response. Default value: true.
       #      - enum_type=string # Serialized type of enumeration type. Use "string" for string-based serialization. The default value is integer.
-      - output_mode=source_relative # Output file generation mode. By default, only one openapi.yaml file is generated in the output folder. Using "source_relative" generates a separate "[inputfile].openapi.yaml" file for each '[inputfile].proto' file. The default is: merged.
+      #      - output_mode=source_relative # Output file generation mode. By default, only one openapi.yaml file is generated in the output folder. Using "source_relative" generates a separate "[inputfile].openapi.yaml" file for each '[inputfile].proto' file. The default is: merged.
+      #      - output_mode=merge
       - fq_schema_naming=true # Schema named whether to add a package name, is true, will add a package name, for example: system. Service. V1. ListDictDetailResponse, otherwise as follows: ListDictDetailResponse. The default value is false.
+    strategy: all
   - local: protoc-gen-go
     out: api/v1/services
     opt: paths=source_relative