Problem

Workspace template updates that occur when dynamic parameters are enabled bypass the stop-before-start logic, causing agent reconnection failures and inconsistent behavior.

Root Cause

PR #18425 introduced stop-before-start logic for template updates, but when dynamic parameters support was added, a new code path was created that bypasses this logic:

site/src/api/api.ts:2331-2338 - Dynamic parameters path directly starts workspace without stopping first:

if (isDynamicParametersEnabled) {
  try {
    return await this.postWorkspaceBuild(workspace.id, {
      transition: "start",  // ❌ No stop first
      template_version_id: activeVersionId,
      rich_parameter_values: newBuildParameters,
    });

site/src/api/api.ts:2370-2384 - Legacy path correctly stops first:

// Stop the workspace if it is already running.
if (workspace.latest_build.status === "running") {
  const stopBuild = await this.stopWorkspace(workspace.id);
  const awaitedStopBuild = await this.waitForBuild(stopBuild);
  // ... handle cancellation
}

Additionally, changeWorkspaceVersion (line 2254) never had the stop-first logic at all, regardless of dynamic parameters.

Why Stop-First is Required

As documented in #17840:

• Template updates regenerate agent tokens
• Without a stop+start, resources with ignore_changes (like prebuilds) don't get recreated
• Old agent tokens become invalid, but the agent container continues using them
• This causes agent disconnection and requires manual workspace restart

Proposed Solution

Apply stop-before-start logic to both:

1. The dynamic parameters path in updateWorkspace
2. The changeWorkspaceVersion function

See #17840 for the original reasoning behind stop-before-start behavior.