feat(developer-knowledge): add Developer Knowledge API samples

[llatif]

Description

Adds Node.js code samples for the Developer Knowledge API using the @google/developer-knowledge client library:

• answerQuery.js (developerknowledge_answer_query) - Answers queries against a developer knowledge corpus.
• getDocument.js (developerknowledge_get_document) - Retrieves a document by resource name.
• batchGetDocuments.js (developerknowledge_batch_get_documents) - Retrieves multiple documents in a single request.
• searchDocumentChunks.js (developerknowledge_search_document_chunks) - Searches document chunks with pagination limit.

Includes comprehensive Mocha unit tests (test/samples.test.js), ESLint/Prettier configuration with license headers, package metadata, README documentation, and added developer-knowledge to CODEOWNERS.

Checklist

• I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
• Tests pass: npm test (see Testing)
• Lint pass: npm run lint (see Style)
• Required CI tests pass (see CI testing)
• These samples need a new API enabled in testing projects to pass (developerknowledge.googleapis.com)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This pull request is from a branch created directly off of GoogleCloudPlatform/nodejs-docs-samples. Not a fork.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new sample directory, and I created GitHub Actions workflow for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[snippet-bot]

Here is the summary of changes.

You are about to add 4 region tags.

• developer-knowledge/answerQuery.js:17, tag developerknowledge_answer_query
• developer-knowledge/batchGetDocuments.js:17, tag developerknowledge_batch_get_documents
• developer-knowledge/getDocument.js:17, tag developerknowledge_get_document
• developer-knowledge/searchDocumentChunks.js:17, tag developerknowledge_search_document_chunks

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[gemini-code-assist]

Code Review

This pull request introduces Node.js code samples and tests for the Google Developer Knowledge API, including scripts for answering queries, retrieving documents, batch-fetching documents, and searching document chunks. The review feedback highlights potential TypeError issues across several files (answerQuery.js, getDocument.js, and searchDocumentChunks.js) where properties or methods are accessed on potentially undefined or null fields (such as response.answer, document.content, and chunk.content). It is recommended to use optional chaining or fallback values to ensure robust error handling.

[gemini-code-assist]

If response.answer is undefined or null (for example, if no answer could be generated for the query), accessing response.answer.answerText or its other properties directly will throw a TypeError. Use optional chaining or default values to safely handle cases where the answer is missing.

Suggested change

	console.log(`Answer:\n${response.answer.answerText}\n`);
	const citationsCount = response.answer.citations
	? response.answer.citations.length
	: 0;
	const referencesCount = response.answer.references
	? response.answer.references.length
	: 0;
	const answerText = response.answer?.answerText || '';
	console.log('Answer:\n' + answerText + '\n');
	const citationsCount = response.answer?.citations?.length || 0;
	const referencesCount = response.answer?.references?.length || 0;

[gemini-code-assist]

If document.content is undefined or null, calling .substring() on it will throw a TypeError. Use a fallback empty string or optional chaining to safely handle missing content.

Suggested change

	console.log(`Content Preview: ${document.content.substring(0, 150)}...\n`);
	const contentPreview = (document.content || '').substring(0, 150);
	console.log('Content Preview: ' + contentPreview + '...\n');

[gemini-code-assist]

If chunk.content is undefined or null, calling .substring() on it will throw a TypeError. Use a fallback empty string or optional chaining to safely handle missing content.

    const contentPreview = (chunk.content || '').substring(0, 100);
    console.log('Content Preview: ' + contentPreview + '...\n');