fix(cli): allow new immutable parameters via --parameter flag during update (#22221)

## Problem

When a template adds a new immutable parameter, `coder update
--parameter param=value` fails with:

```
error: start workspace: parameter "machine_type" is immutable and cannot be updated
```

The interactive prompt handles this correctly (allows setting first-time
immutable params), but the CLI `--parameter` flag path does not.

## Root Cause

In `cli/parameterresolver.go`, `verifyConstraints()` runs before the
interactive prompt and unconditionally rejects any immutable parameter
during updates. It doesn't distinguish between **new** immutable
parameters (first-time use, should be allowed) and **existing** ones
(already set, should be blocked from changing).

## Fix

Added an `isFirstTimeUse` check to the immutable parameter constraint,
matching the logic already used by the interactive prompt path (line
323). New immutable parameters can now be set via `--parameter`, while
existing immutable parameters are still blocked from being changed.

## Testing

Added `TestUpdateValidateRichParameters/NewImmutableParameterViaFlag`
which:
1. Creates a workspace with a mutable parameter
2. Updates the template to add a new immutable parameter
3. Runs `coder update --parameter immutable_param=value`
4. Verifies the update succeeds and the parameter is set correctly

Fixes #22164

cli/parameterresolver.go

@@ -297,7 +297,7 @@ func (pr *ParameterResolver) verifyConstraints(resolved []codersdk.WorkspaceBuil
 			return xerrors.Errorf("ephemeral parameter %q can be used only with --prompt-ephemeral-parameters or --ephemeral-parameter flag", r.Name)
 		}
 
-		if !tvp.Mutable && action != WorkspaceCreate {
+		if !tvp.Mutable && action != WorkspaceCreate && !pr.isFirstTimeUse(r.Name) {
 			return xerrors.Errorf("parameter %q is immutable and cannot be updated", r.Name)
 		}
 	}