Remove is_mamba() check to support conda-libmamba-solver for repoquery command

[RRosio]

This pull request updates the dependency query and package listing logic in EnvManager, improves fallback handling when repoquery is unavailable, and modernizes related tests.

Backend improvements and fallback logic:

• Updated pkg_depends and list_available methods in EnvManager to handle cases where repoquery is unavailable or fails, providing warnings and falling back to conda search as needed.
• Refactored the flow in list_available to check the return code from repoquery and only process results if successful.

Testing improvements:

• Updated multiple tests in test_api.py to simulate and assert correct behavior when repoquery is unavailable, including proper use of fallbacks and correct command invocation
• Added a new async test in test_manager.py to verify that list_available correctly falls back to conda search when repoquery fails.

User interface improvement:

• Updated the error message in the PkgGraph React component to clarify that dependency queries require either conda-libmamba-solver or mamba, and provided a more relevant help link.

[github-actions]

👈 Launch a binder notebook on the branch RRosio/gator/conda-dep-graph