fix: validate return_images support per model (#61)

[Abhishek21g]

Summary

Fixes #61 — clear client-side behavior when return_images=True is used with models that do not return images.

Problem

return_images=True on sonar no longer returns image URLs while sonar-pro does. The SDK silently sent the parameter without guidance.

Fix

• perplexity.lib.model_capabilities matrix for known models
• validate_return_images() raises ReturnImagesUnsupportedError for sonar + return_images=True
• StreamChunk.images field for typed access to API responses
• 6 pytest cases with no live API dependency

Verification

python -m venv .venv && .venv/bin/pip install -e . pytest pytest-asyncio pytest-xdist
.venv/bin/pytest tests/test_model_capabilities.py -v