feat: add Google Gemini provider

[AasheeshLikePanner]

🎯 Description

• Add GeminiProvider with support for all Gemini models (1.5, 2.0, 2.5, 3.x)
• Support for completion, streaming, and tool calling
• Updated pricing for latest models (March 2026):
  • Gemini 3.1 Pro Preview: $2.00/$12.00 per 1M tokens
  • Gemini 3.1 Flash Preview: $0.50/$3.00 per 1M tokens
  • Gemini 3.1 Flash-Lite Preview: $0.25/$1.50 per 1M tokens
  • Gemini 2.5 Pro: $1.25/$10.00 per 1M tokens
  • Gemini 2.5 Flash: $0.30/$2.50 per 1M tokens
  • Gemini 2.5 Flash-Lite: $0.10/$0.40 per 1M tokens
• Automatic retry logic with exponential backoff
• HTTP config support for enterprise SSL/proxy
• Fix agent module export in cascadeflow/init.py

Tested with real API key - working correctly.

🔗 Related Issues

• Related to #

🔄 Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🚀 New provider integration
• 🤖 Tool calling / function support

🧪 Testing

Test cases added

• Manual testing

How to test

from cascadeflow.providers import PROVIDER_REGISTRY
from cascadeflow.providers.gemini import GeminiProvider
import os
os.environ['GEMINI_API_KEY'] = 'your-key'

provider = GeminiProvider()
response = await provider.complete(
    prompt="What is Python?",
    model="gemini-2.5-flash"
)
print(response.content)

📋 Checklist

Code Quality

• My code follows the project's style guidelines
• I have performed a self-review of my code
• My changes generate no new warnings
• I have added type hints where appropriate
• I have run `black` to format my code
• I have run `ruff` and fixed all linting issues
• I have run `mypy` for type checking

Testing

• New and existing unit tests pass locally with my changes
• Tested with real Gemini API key

Breaking Changes

• This PR includes NO breaking changes

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By submitting this PR, I confirm that:

• I have read and followed the CONTRIBUTING.md guidelines
• My contribution is my own original work or properly attributed
• I agree to license my contribution under the project's MIT license
• I have tested my changes thoroughly