thv: Stop on a Foreground Process Removes the Workload

Bug description

Running thv stop on a workload started by thv run --foreground causes the workload to be removed from thv list --all results.

This is perceived as a bug, because stop is effectively acting as rm.

Steps to reproduce

1. thv run --foreground --name fetch-foreground fetch
2. In another shell: thv stop fetch-foreground
3. Observe thv list --all does not contain fetch-foreground

Expected behavior

There are multiple acceptable behaviors:

• thv stop cannot be run on foreground workloads
• thv stop does not remove the state for the foreground workload, but stops it. In other words, thv list --all contains fetch-foreground and it is in a stopped status.

Actual behavior

thv stop behaves like thv rm when executed against foreground workloads.

Environment (if relevant)

• OS/version: Mac Version 15.4.1
• ToolHive version: v0.6.11-42-g2b6a4a76

Additional context

This PR contains significant prior art: https://github.com/stacklok/toolhive/pull/2927#pullrequestreview-3553421250

The problem is the --foreground process acts as the owner of the workload and will always remove the workload when it exits. This can be seen in the logs below:

Reproduce the lifecycle with thv run --foreground --name fetch-foreground fetch, then run thv stop fetch-foreground and inspect thv list --all. Read the prior art in PR #2927 and trace how the foreground owner handles exit. Done means stop either rejects foreground workloads or leaves them listed with a stopped status.