Fix TypeError when outputDir doesn't contain docPath

[kkman021]

Fix undefined reference error in basePath calculation when outputDir.split(docPath)[1] returns undefined. This
occurs when using multiple docs plugin instances with mismatched path configurations.

Add fallback to empty string to prevent .replace() call on undefined value.

Description

This PR fixes a TypeError that occurs during sidebar generation when outputDir.split(docPath)[1] returns
undefined. The fix adds a fallback to an empty string using the logical OR operator (|| ""), preventing
the subsequent .replace() call from being invoked on undefined.

Changed Line (index.ts:129):

// Before
const basePath = docPath
  ? outputDir.split(docPath!)[1].replace(/^\/+/g, "")
  : outputDir.slice(outputDir.indexOf("/", 1)).replace(/^\/+/g, "");

// After
const basePath = docPath
  ? (outputDir.split(docPath!)[1] || "").replace(/^\/+/g, "")
  : outputDir.slice(outputDir.indexOf("/", 1)).replace(/^\/+/g, "");

Motivation and Context

When configuring multiple @docusaurus/plugin-content-docs instances with different paths alongside
docusaurus-plugin-openapi-docs, the plugin crashes with:

TypeError: Cannot read properties of undefined (reading 'replace')
    at groupByTags (node_modules/docusaurus-plugin-openapi-docs/lib/sidebars/index.js:85:38)

This occurs because outputDir.split(docPath)[1] returns undefined when the outputDir path doesn't contain
docPath as expected. The code then attempts to call .replace() on undefined, causing the error.

Example configuration that triggers the bug:
plugins: [
  [
    '@docusaurus/plugin-content-docs',
    {
      id: 'WEDAApi',
      path: 'Weda/Api',
      routeBasePath: 'apis/weda',
      // ...
    },
  ],
  [
    'docusaurus-plugin-openapi-docs',
    {
      config: {
        weda: {
          specPath: "src/api/weda.yml",
          outputDir: "Weda/Api",
          sidebarOptions: {
            groupPathsBy: "tag",
          },
        }
      }
    }
  ],
]

The fix allows the plugin to gracefully handle mismatched path configurations by defaulting to an empty string
 when the split operation fails.

How Has This Been Tested?

Testing Environment:
- Node.js: v22.14.0
- Docusaurus: 3.9.1
- docusaurus-plugin-openapi-docs: 4.5.1
- OS: Windows 11

Test Steps:
1. Configured multiple docs plugin instances with different paths
2. Set up OpenAPI docs plugin with sidebarOptions.groupPathsBy: "tag"
3. Ran npx docusaurus gen-api-docs weda
4. Verified successful sidebar generation without errors
5. Started dev server with npm start
6. Confirmed API documentation displays correctly with proper tag-based categorization

Test Results:
- ✅ Sidebar generation completes without TypeError
- ✅ Tag-based sidebar structure generated correctly
- ✅ All API endpoints accessible and properly categorized
- ✅ Development server starts without errors

Before Fix: Command failed with TypeError: Cannot read properties of undefined (reading 'replace')

After Fix: All commands execute successfully, sidebar generated with 90+ API endpoints properly organized by
tags.

Screenshots (if appropriate)

N/A - This is a bug fix with no visual changes to the UI.

Types of changes

- Bug fix (non-breaking change which fixes an issue)