fix: avoid implicit bundle target for non-bundle auth resolution

[jmahotiedu]

## Summary

• Stop loading bundle auth configuration implicitly in MustWorkspaceClient unless --target / --environment is explicitly set.

• Keep explicit target behavior unchanged so bundle context remains opt-in.

• Add regression tests covering:

  • Non-bundle command inside bundle directory respects DATABRICKS_HOST.
  • Explicit --target still uses bundle target host.

## Problem
Fixes #4502. Non-bundle commands such as databricks current-user me were resolving authentication from the bundle’s default target when run inside a bundle directory, potentially overriding DATABRICKS_HOST.

## Testing

• go test ./cmd/root

[github-actions]

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Trigger:
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Inputs:

• PR number: 4504
• Commit SHA: db5cb1821e7e05c06d87783b389b90a59f630f67

Checks will be approved automatically on success.

[NateWerner]

I just recently had a 1 hour troubleshooting session trying to figure out why an Azure SP could not login to a test environment when it worked in dev in my Github Workflow, when I finally realized it was not actually logging into test, but following the bundle's default environment. In my case I have a utility workflow script (running in a folder that also has DAB) that is doing non-bundle related items with CLI that was relying on my workflow's DATABRICKS_HOST env var.
Feels odd to spray "-t " all over my non-bundle Databricks utility script.
Thanks for proposing this change.

[jmahotiedu]

Thanks @NateWerner, this is exactly the real-world failure mode this PR is intended to fix.

With this change, bundle auth is only used when --target / --environment is explicitly provided. For non-bundle commands, even inside a DAB directory, auth resolution respects DATABRICKS_HOST; explicit -t behavior remains unchanged.

@andrewnester if you have bandwidth, could you please approve workflow execution for #4504 so required checks can run? Happy to make any follow-up changes.