fix(agent-loader): report the actual root_agent type mismatch

[Diwak4r]

What

Fixes #6606: AgentLoader no longer discards its own diagnosis when a module's root_agent has the wrong type.

The bug

When a module exposes a root_agent that is not a BaseAgent/BaseNode (for example an App assigned to root_agent instead of app), the loader detected the exact problem, formatted it with the offending type, wrote it to logger.warning, and then fell through to raise the generic "No root_agent found" error.

Every actionable statement in that generic error is wrong for this failure:

• root_agent was found, it exists in the module.
• The directory structure was correct.
• The cwd was correct; the HINT tells the user to restructure a working layout.

The fix

• _load_from_module_or_package and _load_from_submodule now remember the mismatch (location, wrong_type) instead of throwing it away.
• _perform_load raises the precise error when a mismatch was recorded and no pattern produced a valid load:

Found 'judge.agent.root_agent' in module, but it is a App, not a BaseAgent or BaseNode. If you meant to export an App, name it `app` instead of `root_agent`.

• The mismatch state is reset per load, so caching/reuse across agents is unaffected.
• The fallback behavior is preserved: if a later pattern (e.g. agent.py with a correct root_agent) loads successfully, the mismatch is never surfaced.

Tests

Added two unit tests covering both dispatch paths:

• test_agent_with_wrong_root_agent_type_error — module file with root_agent = App(...).
• test_agent_submodule_wrong_root_agent_type_error — {agent}/agent.py with root_agent = App(...).

Both assert the precise diagnosis appears and that the misleading "No root_agent found" path is not taken. Full suite: tests/unittests/cli/utils/test_agent_loader.py 41 passed, test_nested_agent_loader.py 7 passed.