feat: add gapps-gemini-client library with initial implementation (#264)

- Introduced `GeminiClient` class for interacting with Google's Gemini AI API.
- Added methods for text generation, structured JSON output, and conversation management.
- Created `package.json` and `README.md` with project details and usage instructions.
- Included comprehensive tests for functionality and edge cases.

Author: robaone-redshelf

projects/gapps-gemini-client/README.md

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+# gapps-gemini-client
+
+A Google Apps Script client library for interacting with Google's Gemini AI API. This library provides a clean, feature-rich interface for generating text, structured JSON responses, and managing conversational AI interactions within Google Apps Script projects.
+
+## Features
+
+- **Text Generation**: Generate plain text responses from prompts
+- **Structured JSON Output**: Force JSON responses with optional schema validation
+- **Conversation Management**: Maintain conversation history with user/model turns
+- **System Instructions**: Set persistent system-level instructions
+- **Model Selection**: Switch between different Gemini models (e.g., gemini-1.5-flash, gemini-1.5-pro)
+- **Automatic Retries**: Built-in retry logic with exponential backoff for transient errors
+- **Character Budget Management**: Automatic conversation history trimming to stay within token limits
+- **Comprehensive Testing**: Extensive test suite using the gapps-unit-test-library
+
+## Dependencies
+
+This project depends on the **gapps-unit-test-library** for its comprehensive test suite. The testing framework provides:
+- Test class structure with `before()` and `after()` hooks
+- Assertion utilities (`Assert.isTrue`, `Assert.match`, `Assert.deepEquals`)
+- Automated test discovery and execution
+
+## Setup
+
+1. **API Key Configuration**: Set your Gemini API key as a Script Property named `gemini.key`
+2. **Include Dependencies**: Ensure both `GeminiClient.js` and the `Test.js` from gapps-unit-test-library are included in your Google Apps Script project
+
+## Basic Usage
+
+### Simple Text Generation
+
+```javascript
+const client = new GeminiClient(PropertiesService.getScriptProperties().getProperty('gemini.key'));
+
+// Generate plain text
+const response = client.generateText('Write a 2-sentence fun fact about Belize.', { 
+  temperature: 0.7 
+});
+console.log(response);
+```
+
+### Structured JSON Output
+
+```javascript
+const schema = {
+  type: 'array',
+  items: {
+    type: 'object',
+    properties: {
+      district: { type: 'string' },
+      attraction: { type: 'string' },
+      tags: { type: 'array', items: { type: 'string' } }
+    },
+    required: ['district', 'attraction']
+  }
+};
+
+const response = client.generateJSON(
+  'Return 3 Belize travel attractions as JSON array of {district, attraction, tags}.',
+  schema,
+  { temperature: 0.2 }
+);
+
+const attractions = JSON.parse(response);
+```
+
+### Conversational AI
+
+```javascript
+// Initialize with conversation history
+const client = new GeminiClient(apiKey, {
+  history: [
+    { role: 'user', parts: [{ text: 'Plan a 1-day Belize City itinerary.' }] }
+  ],
+  systemInstruction: 'Be concise, friendly, and factual.'
+});
+
+// Send message and get response
+const response = client.send({ temperature: 0.4 });
+
+// Add follow-up
+client.addTurn('user', 'Add a family-friendly lunch spot.');
+const followUpResponse = client.send({ temperature: 0.4 });
+```
+
+## API Reference
+
+### Constructor
+
+```javascript
+new GeminiClient(apiKey, opts = {}, fetch = UrlFetchApp.fetch)
+```
+
+**Parameters:**
+- `apiKey` (string): Your Gemini API key
+- `opts` (object, optional):
+  - `model` (string): Gemini model name (default: 'gemini-1.5-flash')
+  - `timeoutMs` (number): Request timeout in milliseconds (default: 60000)
+  - `history` (array): Initial conversation history
+  - `systemInstruction` (string): System-level instruction
+- `fetch` (function): Fetch function for testing (default: UrlFetchApp.fetch)
+
+### Core Methods
+
+#### `generateText(prompt, generationConfig = {})`
+Generate plain text from a prompt.
+
+#### `generateJSON(prompt, schema = undefined, config = {})`
+Generate structured JSON output with optional schema validation.
+
+#### `send(generationConfig = {})`
+Send the current conversation and get model reply. Appends the response to history.
+
+#### `sendJSON(schema = undefined, config = {})`
+Send conversation and force JSON response with optional schema.
+
+### Conversation Management
+
+#### `addTurn(role, text)`
+Add a user or model turn to conversation history.
+
+#### `clearHistory()`
+Clear all conversation history.
+
+#### `truncateHistory(maxParts = 20)`
+Trim history to keep last N parts.
+
+#### `ensureCharBudget(maxChars = 12000)`
+Ensure history stays under character budget (rough proxy for tokens).
+
+### Configuration
+
+#### `setModel(model)`
+Switch to a different Gemini model.
+
+#### `setSystemInstruction(text)`
+Set a persistent system instruction.
+
+## Testing
+
+The library includes comprehensive tests that demonstrate proper usage:
+
+- **Basic Functionality**: API key validation, text generation, JSON responses
+- **Conversation Management**: History management, turn handling, system instructions
+- **Error Handling**: Retry logic, invalid responses, edge cases
+- **Configuration**: Model switching, character budget management
+
+Run tests using the provided test functions:
+- `test_GeminiClient()` - Basic functionality tests
+- `test_GeminiClientChat()` - Conversation management tests  
+- `test_GeminiClientChatEdgeCases()` - Edge case and error handling tests
+
+## Error Handling
+
+The client includes robust error handling:
+- Automatic retries with exponential backoff for transient errors
+- Validation of API responses and JSON output
+- Clear error messages for common issues
+- Graceful handling of empty conversation history
+
+## Examples
+
+See the test classes in the source code for comprehensive usage examples covering all features and edge cases.
+
+

projects/gapps-gemini-client/package.json

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+{
+  "name": "gapps-gemini-client",
+  "version": "0.1.0",
+  "scripts": {
+    "build": "echo \"Not yet implemented\"",
+    "test": "echo \"Not yet implemented\"",
+    "test:e2e": "echo \"Not yet implemented\"",
+    "deploy:pr": "echo \"Not yet implemented\"",
+    "deploy:develop": "echo \"Not yet implemented\"",
+    "deploy:staging": "echo \"Not yet implemented\"",
+    "deploy:production": "echo \"Not yet implemented\""
+  },
+  "devDependencies": {
+  },
+  "dependencies": {
+  }
+}