Allow parameters to opt out of isolated user-secrets copying

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

`aspire run --isolated` copies the AppHost user secrets into an isolated user-secrets store. Some parameter values should not be copied, such as credentials or tokens that must be unique to each instance. Today there is no way for an AppHost author to identify those parameters, so the isolated instance inherits values that should be supplied or generated independently.

### Describe the solution you'd like

Add a way to mark a parameter as not copyable to an isolated user-secrets store. When isolated mode creates its user-secrets copy, the value associated with that parameter should be omitted. The isolated instance should then resolve the parameter through the normal missing-value flow instead of receiving the original value.

### Additional context

I searched the existing issues and did not find one covering a per-parameter opt-out from isolated user-secrets copying.

Locate the implementation of `aspire run --isolated` and the code that copies AppHost user secrets into the isolated store. Trace how parameter values are written and how missing values are resolved, then confirm that an opted-out parameter is omitted and follows the normal missing-value flow.

Written by the indexing model from the issue text.