Fix OAuth callbacks and extension module loading

[xlr8harder]

Summary

This fixes two harn runtime issues found while dogfooding harn-gibson as a harn extension:

• Pass OAuth login callbacks as an attribute-style callback object instead of a dict.
• Register extension modules in sys.modules before executing them, matching normal import behavior.

Why

Interactive /login for the openai-codex provider currently fails because the OAuth provider expects callbacks.onAuth, but interactive mode passes {"onAuth": ...}. This produces:

'dict' object has no attribute 'onAuth'

Separately, extension files are executed via exec_module() without being inserted into sys.modules. Some standard Python machinery, including dataclasses with postponed annotations, expects sys.modules[cls.__module__] to exist during class creation. Without that, valid extension code can fail during load.

Changes

• Updated interactive login callback construction to use SimpleNamespace.
• Updated extension loading to insert the module into sys.modules before exec_module().
• Restores the prior sys.modules entry, or removes the temporary entry, if extension execution fails.
• Added regression coverage for both behaviors.

Testing

Ran targeted regressions:

uv run --no-sync --all-packages pytest tests/test_coding_agent/test_interactive_mode_foundation.py::test_show_login_dialog_only_collects_manual_code_for_callback_providers tests/test_coding_agent/ test_resources_and_extensions.py::test_extension_loader_registers_modules_for_dataclass_annotations -q

Result: 2 passed