Remove useless condition in example

## Motivation and Context

The example files `examples/http_client.rb` and `examples/streamable_http_client.rb` are
intended to be run as standalone scripts and are not designed to be reused as modules (e.g., via `require`).
The `if __FILE__ == $PROGRAM_NAME` guard therefore serves no purpose and is removed.

## How Has This Been Tested?

Manually verified that each example script can still be executed directly
(`ruby examples/http_client.rb`, `ruby examples/streamable_http_client.rb`).
No automated tests cover files under `examples/`.

## Breaking Changes

None. The files under `examples/` are standalone scripts and are not part of the gem's public API.

examples/http_client.rb

@@ -137,8 +137,7 @@ class MCPHTTPClient
   end
 end
 
-# Main script
-if __FILE__ == $PROGRAM_NAME
+def main
   puts <<~MESSAGE
     MCP HTTP Client Example
     Make sure the HTTP server is running (ruby examples/http_server.rb)
@@ -182,3 +181,5 @@ if __FILE__ == $PROGRAM_NAME
     client.close_session
   end
 end
+
+main

examples/streamable_http_client.rb

@@ -176,4 +176,4 @@ def main
   end
 end
 
-main if __FILE__ == $PROGRAM_NAME
+main