Add custom guardrail support

[lleonardo-franco]

Summary

• Add a function-based CustomGuardrail wrapper for domain-specific guardrail checks.
• Allow TealOpenAIConfig to register custom guardrails alongside the existing guardrail engine.
• Export the new public guardrail types and cover request blocking behavior.

Validation

• /private/tmp/tealtiger-python-venv/bin/python -m pytest tests/test_guardrail_engine.py tests/clients/test_teal_openai.py -q

Refs agentguard-ai/tealtiger#7

[nagasatish007]

Merged! Nice feature @lleonardo-franco.

Could you open a follow-up PR adding documentation for this? Users won't discover it without docs. Something like:

A "Custom Guardrails" section in the README with a quick example
An example file at
custom_guardrail.py
showing 2-3 real-world use cases (e.g., medical terms blocker, financial compliance check, domain-specific content filter)
The API is clean — just needs visibility. Thanks!