Fix tracking URI to explicitly append /api to map to REST route

[aryawadhwa]

Resolves #1075

Description

When users provide an MLFLOW_TRACKING_URI associated with remote services like DagsHub (e.g., https://dagshub.com/username/repo.mlflow), the underlying HTTP requests currently fail due to 404 HTML redirects.

This occurs because standard MLflow REST API calls expect the /api route, but MLFlowClient.jl currently enforces the raw URI provided in ENV["MLFLOW_TRACKING_URI"] without validating or appending the required /api suffix.

Changes Made

• Updated the MLFlow constructor in src/types/mlflow.jl to gracefully append /api if missing when parsing tracking URIs from the environment.
• Left the existing Base64 authentication credential logic intact, as the credentials themselves were routing perfectly.
• Added a new @testset in test/types/mlflow.jl rigorously unit testing the tracking URI manipulation against multiple edge cases (missing slash, trailing slash, already contains /api, etc.).

Testing

Ran Pkg.test() locally and all tests gracefully pass while successfully shielding the downstream runtests.jl environment state.