external-agent-migration: avoid mixed MCP transport configs

[stefanstokic-oai]

Why

MCP migration could recursively merge an imported server into an existing same-named Codex server. When one definition used stdio and the other used HTTP, this produced an invalid mixed configuration containing both command and url.

What changed

• Merge MCP configuration at the server level instead of field by field.
• Preserve an existing same-named Codex MCP server unchanged.
• Report only MCP servers that would actually be added during detection.
• Add regression coverage for mixed command/HTTP source configurations.
• Use neutral fixture names and reserved example.com URLs.

Test plan

• just test -p codex-app-server repo_mcp
  • 5 tests passed.
• just test -p codex-external-agent-migration mcp_migration_prefers_command_transport_for_mixed_server_config
  • 1 test passed.