Fix assertion error when plugin unload fails

Return early from reloadPluginFromFile when DynamicPlugins.unloadPlugin()
returns false, instead of attempting to load the new plugin. This prevents
the assertion in PluginSet.withPlugin() which expects any existing plugin
to have isMarkedForLoading=false before loading a new version.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: jonnyzzz

src/main/kotlin/com/jonnyzzz/intellij/hotreload/PluginHotReloadService.kt

@@ -167,12 +167,24 @@ class PluginHotReloadService {
                 if (!unloaded) {
                     val warnMsg = HotReloadBundle.message("error.unload.failed")
                     log.warn(warnMsg)
-                    progress.report(warnMsg)
+                    progress.reportError(warnMsg)
 
                     // Create memory dump if unload failed
                     if (MemoryDumpHelper.memoryDumpAvailable()) {
                         memoryDumpPath = createMemoryDump(pluginIdString, progress)
                     }
+
+                    // Return early - cannot proceed with loading new plugin
+                    // when old plugin is still loaded (would cause assertion in PluginSet.withPlugin)
+                    return ReloadResult(
+                        success = false,
+                        message = warnMsg,
+                        pluginId = pluginIdString,
+                        pluginName = existingPlugin.name,
+                        restartRequired = true,
+                        memoryDumpPath = memoryDumpPath,
+                        unloadBlockedReason = unloadBlockedReason
+                    )
                 } else {
                     progress.report(HotReloadBundle.message("progress.unloaded"))
                 }