refactor(installer): reduce deep nesting in installShippedApps()

[joshtrichards]

• Related to Consolidate app management code #8505 (app management code consolidation)

Summary

• Extracts nested directory scanning and app filtering logic into dedicated helper methods, reducing nesting from ~7 levels to 2-3, greatly improving readability/maintainability.
• Modernizes directory traversal to use scandir() instead of opendir/readdir/closedir pattern.
• Some additional helpful comments sprinkled throughout

No functional changes - preserves exact same behavior including error handling for TableExistsException during upgrades.

Before:

		foreach (\OC::$APPSROOTS as $app_dir) {
			if ($dir = opendir($app_dir['path'])) {
				while (false !== ($filename = readdir($dir))) {
					if ($filename[0] !== '.' && is_dir($app_dir['path'] . "/$filename")) {
						if (file_exists($app_dir['path'] . "/$filename/appinfo/info.xml")) {
							if ($this->config->getAppValue($filename, 'installed_version') === '') {
								$enabled = $this->appManager->isDefaultEnabled($filename);
								if (($enabled || in_array($filename, $this->appManager->getAlwaysEnabledApps()))
									  && $this->config->getAppValue($filename, 'enabled') !== 'no') {
									if ($softErrors) {
										try {
											$this->installShippedApp($filename, $output);
										} catch (HintException $e) {
											if ($e->getPrevious() instanceof TableExistsException) {
												$errors[$filename] = $e;
												continue;
											}
											throw $e;
										}
									} else {
										$this->installShippedApp($filename, $output);
									}
									$this->config->setAppValue($filename, 'enabled', 'yes');
								}
							}
						}
					}
				}
				closedir($dir);
			}
		}

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)