docs(js/samples): add readmes for all samples

[yesudeep]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @yesudeep, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the documentation for JavaScript test applications by introducing detailed READMEs for a wide array of examples, covering various Genkit features from basic generation to advanced multi-agent systems and multimodal RAG. Concurrently, it unifies the environment variable used for Gemini API keys, streamlining the setup process for developers.

Highlights

• Documentation Expansion: Added comprehensive README files for 16 new JavaScript test applications, detailing their features, setup, and testing instructions.
• API Key Standardization: Standardized the API key environment variable from GOOGLE_GENAI_API_KEY to GEMINI_API_KEY across all relevant documentation, including existing samples and newly added test apps.

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Changelog

• js/testapps/basic-gemini/README.md
  • Added a new README for the Basic Gemini test application, outlining its features, setup, and testing steps.
• js/testapps/context-caching/README.md
  • Updated the API key environment variable from GOOGLE_GENAI_API_KEY to GEMINI_API_KEY.
• js/testapps/context-caching2/README.md
  • Updated the API key environment variable from GOOGLE_GENAI_API_KEY to GEMINI_API_KEY.
• js/testapps/dev-ui-gallery/README.md
  • Added a new README for the Dev UI Gallery test application, demonstrating Genkit Dev UI features.
• js/testapps/durable-streaming/README.md
  • Added a new README for the Durable Streaming test application, showcasing durable streaming with various backends.
• js/testapps/esm/README.md
  • Added a new README for the ESM Compatibility Test application, verifying ESM imports for Genkit packages.
• js/testapps/express/README.md
  • Added a new README for the Express Integration test application, demonstrating Genkit flows with Express.js.
• js/testapps/flow-sample1/README.md
  • Added a new README for the Flow Fundamentals test application, illustrating core Genkit flow primitives.
• js/testapps/flow-simple-ai/README.md
  • Added a new README for the Flow + AI Kitchen Sink test application, exercising a wide range of Genkit features.
• js/testapps/format-tester/README.md
  • Added a new README for the Format Tester application, providing an automated test harness for output formats.
• js/testapps/model-armor/README.md
  • Added a new README for the Model Armor test application, demonstrating content safety filtering.
• js/testapps/model-tester/README.md
  • Added a new README for the Model Tester application, featuring an automated conformance test suite for models.
• js/testapps/multiagents-demo/README.md
  • Added a new README for the Multi-Agent Demo application, showcasing a multi-agent architecture.
• js/testapps/multimodal/README.md
  • Added a new README for the Multimodal RAG test application, demonstrating multimodal RAG with PDFs and videos.
• js/testapps/next/README.md
  • Added a new README for the Next.js Integration test application, testing the @genkit-ai/next plugin.
• js/testapps/prompt-file/README.md
  • Added a new README for the Dotprompt Files test application, demonstrating Genkit's Dotprompt system.
• js/testapps/rag/README.md
  • Added a new README for the RAG test application, demonstrating RAG workflows with multiple vector store backends.
• js/testapps/vertexai-modelgarden/README.md
  • Added a new README for the Vertex AI Model Garden test application, showcasing third-party models via Model Garden.
• samples/js-angular/README.md
  • Updated the API key environment variable from GOOGLE_GENAI_API_KEY to GEMINI_API_KEY.
• samples/js-character-generator/README.md
  • Updated the API key environment variable from GOOGLE_GENAI_API_KEY to GEMINI_API_KEY.
• samples/js-coffee-shop/README.md
  • Updated the API key environment variable from GOOGLE_GENAI_API_KEY to GEMINI_API_KEY.
• samples/js-prompts/README.md
  • Updated the API key environment variable from GOOGLE_GENAI_API_KEY to GEMINI_API_KEY.
• samples/js-schoolAgent/README.md
  • Updated the API key environment variable from GOOGLE_GENAI_API_KEY to GEMINI_API_KEY.

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[gemini-code-assist]

Code Review

This pull request adds comprehensive README files for many of the JavaScript sample and test applications, and standardizes the Gemini API key environment variable to GEMINI_API_KEY across existing documentation. The new documentation is a great addition and significantly improves the usability of the samples. My review includes a few minor suggestions to improve consistency in the new README files, particularly around model version numbers and placeholder text.