fix(npm): repair .cmd wrappers for already-installed tools on Windows

EnsureInstalledFromSpecs skips installation when a tool is already
present (e.g. restored from a CI cache). If the cached installation
was created before PostInstall was implemented, the npm .cmd wrappers
still contain the broken relative node.exe path.

Add an idempotent PostInstall repair pass inside the 'already installed'
branch for npm tools on Windows. PostInstall reads and rewrites .cmd
files to use the absolute node.exe path — safe to call repeatedly.
On Linux/macOS PostInstall is a no-op, so there is no performance impact
on those platforms.

Author: snowdream

internal/service/installation.go

@@ -974,6 +974,21 @@ func (im *InstallationManager) EnsureInstalledFromSpecs(ctx context.Context, too
 		// Check if already installed using robust variant detection
 		installed, _ := im.IsInstalled(ctx, toolName, version, backendName)
 		if installed {
+			// On Windows, npm-generated .cmd wrappers may have been restored from
+			// a cache that was built before PostInstall was implemented. Re-run
+			// PostInstall unconditionally for npm tools on Windows — it is idempotent
+			// (reads & rewrites .cmd files) and a no-op on non-Windows platforms.
+			if runtime.GOOS == "windows" && backendName == "npm" {
+				p := im.providerRegistry.GetWithBackend(toolName, backendName)
+				fsName := env.GetFSToolName(toolName, backendName)
+				installPath := filepath.Join(env.GetInstallsDir(), fsName, version)
+				if postErr := p.PostInstall(ctx, toolName, installPath, version); postErr != nil {
+					logger.Debug("npm PostInstall repair skipped", map[string]interface{}{
+						"tool":  toolName,
+						"error": postErr.Error(),
+					})
+				}
+			}
 			continue
 		}