fix(aiplatform): update Gemini model from 2.0-flash to gemini-2.5-flash

[angelcaamal]

fix(aiplatform): update Gemini models and rewrite image generation sample

Description

This PR updates the model references and APIs used across the .NET AI Platform samples (aiplatform/api/AIPlatform.Samples) and integration tests to align with current Vertex AI standards:

• Gemini Models Update: Upgrades text and multimodal model references across all samples and tests from gemini-2.0-flash-001 to gemini-2.5-flash.
• Image Sample Refactor: Rewrites GenerateImage.cs to use the modern Generative AI API (GenerateContentAsync) and the gemini-2.5-flash-image model. This replaces the legacy PredictAsync API pattern and JSON-encoded structures.

Reason for Update:

• gemini-2.0-flash-001 reached end-of-life and was decommissioned on June 1, 2026.
• The legacy imagegeneration@006 model was also decommissioned, causing NotFound RPC errors in CI (GenerateImageTest.TestGenerateImageAsync). Migrating to gemini-2.5-flash-image via the GenerateContent API ensures active service availability and fixes the CI pipeline.

Fixes Internal: b/542706371

[gemini-code-assist]

Code Review

This pull request updates the default model parameter from 'gemini-2.0-flash-001' to 'gemini-2.5-flash' across multiple C# sample files in the AI Platform samples directory. I have no feedback to provide as there are no review comments.

[Kef131]

Update deprecated gemini model correctly.