Register model args issue

[davidrsch]

This PR fixes a bug where tuning compile_optimizer or compile_loss would fail because the tune framework was incorrectly looking for their corresponding dials functions in the dials namespace instead of the kerasnip namespace.

The key changes are:

• R/register_model_args.R:
  • The logic for registering model arguments has been corrected to first check for a full argument name match (e.g., compile_optimizer) in the keras_dials_map before attempting to match a base name (e.g., units).
  • A condition has been added to correctly set the pkg to "kerasnip" for the custom dials functions (optimizer_function and loss_function_keras) defined in this package.
• tests/testthat/test_e2e_fit_compile_tuning.R:
  • A comprehensive end-to-end test has been added to verify that all core fit_* and compile_* arguments can be tuned simultaneously.
  • This test uses the dials update() function and the custom optimizer_function and loss_function_keras functions to ensure the fix is working as expected.

Closes #24

[codecov]

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