Add full_text contents option

[LiamHz]

Summary

• add full_text to ContentsOptions and validation so it serializes to fullText
• expose returned fullText as result.full_text across sync and async responses
• update README and offline coverage for search, search_and_contents, and get_contents

Testing

• uv run pytest tests/unit/test_search_api.py -q
• uv run python -m py_compile exa_py/api.py tests/unit/test_search_api.py
• git diff --check