Fast Environment Configuration Updates

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

When adding existing services to an apphost, you sometimes need to do multiple iterations on the environment variables until you get them just right.
Also, sometime you want to test what happens when only this service changes an environment variable while the system is running (my example is we have an env var of cache version and I want to see that it doesn't access the old cache)

### Describe the solution you'd like

I would like a way in the dashboard, aspire cli, aspire mcp, to be able to add / remove environment variables from a service without having to restart the apphost - only the specific service involved.

### Additional context

I do not expect this to support dynamic values like dynamic urls to other services.
The information on which variables was added should be accessible during the run of the apphost via UI, logs etc. but it doesn't have to be persisted.

Start by mapping the dashboard, Aspire CLI, and Aspire MCP entry points involved in service environment configuration. Define how add/remove operations target only the selected running service, how the change is surfaced in UI or logs, and how dynamic service URLs remain excluded. Done means these updates work without restarting the apphost and are not persisted.

Written by the indexing model from the issue text.