docs: add Anthropic observe() demo to observe_basic example

[amanishimwe]

Pull Request

Description

Extends examples/observe_basic.py with an Anthropic/Claude observe() demo using a mock client (no API key). Keeps the existing OpenAI demo and shows cost tracking plus freeze() / unfreeze() for both providers.

Fixes agentguard-ai/tealtiger#366

Type of Change

• Documentation update

Changes Made

• Added Anthropic mock client and demo_anthropic() to examples/observe_basic.py
• Runs OpenAI and Anthropic demos from the same example script

Testing

• Manual testing performed
• Ran python examples/observe_basic.py (both sections: call → cost → blocked → unfreeze)
• Ran ruff check examples/observe_basic.py

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)

None.

Additional Notes

Docs/example-only change. No runtime API changes.

[amanishimwe]

@nagasatish007, when you get time, kindly review this one as well. It fixes agentguard-ai/tealtiger#366.