fix: report root_agent type mismatch instead of 'No root_agent found'

[loponly]

Fixes #6606

Problem

When a module defines root_agent with a non-agent type, AgentLoader
logs the type mismatch at WARNING but then raises the generic
ValueError: No root_agent found, whose directory-structure guidance
sends users in the wrong direction when their layout is correct.

Fix

_load_from_module_or_package and _load_from_submodule record each
type mismatch; _perform_load raises a targeted ValueError naming the
module and the actual type at its final failure point. When the object
is an App, the error suggests exporting it under the name app.
Fallthrough behavior is preserved: a mismatch in one pattern does not
block a later pattern from loading successfully, and the generic
not-found message is unchanged for the genuinely-missing case.

Testing Plan

• New unit tests in tests/unittests/cli/utils/test_agent_loader.py:
  • test_wrong_type_root_agent_raises_targeted_error
  • test_app_exported_as_root_agent_suggests_app_name
  • test_wrong_type_root_agent_in_agent_module_raises_targeted_error
  • test_valid_agent_module_wins_over_mistyped_package_root_agent
• pytest tests/unittests/cli -n auto: 774 passed, 5 skipped, 4 xfailed, 376 warnings in 84.37s (0:01:24)
• tox (full multi-version matrix): py310 OK, py312 OK, py313 OK, py314 OK.
  py311 has one failure in
  tests/unittests/sessions/migration/test_migration.py::test_migrate_from_sqlalchemy_pickle_preserves_nested_safe_actions_pickle
  that is unrelated to this change and reproduces identically on main:
  the migration unpickler's _ALLOWED_PICKLE_GLOBALS blocks
  builtins.getattr only in the py311 environment (the one where the
  crewai extras install per the pyproject markers). This diff touches
  only cli/utils/agent_loader.py and its test file. Filed separately.

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[loponly]

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