Handle message when the role is developer when using OpenAI provider with instructions

[gokul-312]

Summary

Extends MessageType to drop messages with "developer" role, consistent with how "system" messages are handled. Developer messages are filtered out during message casting, as they are replaced by Instructions in the common format.

Changes

• Modified lib/active_agent/providers/common/messages/_types.rb:

  • Updated the case statement to include "developer" alongside "system" when dropping messages
  • Both roles now return nil during casting

• Added test in test/providers/common/messages/types_test.rb:

  • New test case: "MessageType drops developer messages"
  • Verifies that developer role messages are properly dropped (return nil)

Behavior

When MessageType#cast receives a hash with role: "developer", it now returns nil, matching the behavior for role: "system". This ensures developer messages are filtered out and replaced by Instructions, maintaining consistency with the common message format.

Closes #285