docs: add Gemini and Mistral observe() demos to observe_basic

[amanishimwe]

Pull Request

Description

Extends examples/observe_basic.py with mock Gemini and Mistral observe(). No API key required. Shows cost tracking and freeze() / unfreeze() for each provider.
Fixes agentguard-ai/tealtiger#366

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Code refactoring
• Test improvements

Changes Made

• Added mock Gemini client (generate_content + usage_metadata) and demo_gemini()
• Added mock Mistral client (class name contains mistral, callable chat) and demo_mistral()
• Updated module docstring and __main__ to run OpenAI, Anthropic, Gemini, and Mistral demos

Testing

Describe the tests you ran to verify your changes:

• Unit tests pass (pytest)
• Type checking passes (mypy src)
• Linting passes (ruff check src tests)
• Manual testing performed
• Added new tests for new functionality

Confirmed all four demos print a response, cost summary, and freeze-block message.

Checklist

• My code follows the project's code style
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published

Breaking Changes

None.

Screenshots (if applicable)

N/A — CLI example output only.

Additional Notes

• Mocks follow provider detection in provider_detector.py (Gemini: generate_content; Mistral: class name + callable chat).
• Example-only change; no SDK runtime behavior modified.
• Branch: docs/add-observe-gemini-mistral-example

[amanishimwe]

@nagasatish007, are there any issues with this PR that you would like me to address? Take note, the CLI issues are not introduced by my changes