feat: Add live API integration tests

[aksg87]

Description

Adds comprehensive live API integration tests for Gemini and OpenAI providers to ensure langextract works correctly with real APIs.

This PR introduces a new test suite that validates extraction functionality against actual LLM providers, with proper retry mechanisms for transient errors, cost controls, and CI integration.

Related to #13 (adds test coverage that exposes the tokenizer issue)

Choose one: Testing

How Has This Been Tested?

Tested locally with both providers:

$ tox -e live-api
live-api: commands[0]> pytest tests/test_live_api.py -v -m live_api --maxfail=1
========================== test session starts ==========================
collected 5 items                                                       

tests/test_live_api.py::TestLiveAPIGemini::test_medication_extraction PASSED              [ 20%]
tests/test_live_api.py::TestLiveAPIGemini::test_medication_relationship_extraction PASSED [ 40%]
tests/test_live_api.py::TestLiveAPIGemini::test_multilingual_medication_extraction XFAIL  [ 60%]
tests/test_live_api.py::TestLiveAPIOpenAI::test_medication_extraction PASSED              [ 80%]
tests/test_live_api.py::TestLiveAPIOpenAI::test_medication_relationship_extraction PASSED [100%]

========================= 4 passed, 1 xfailed in 12.34s =========================

Checklist:

• I have read and acknowledged Google's Open Source Code of conduct.
• I have read the Contributing page, and I either signed the Google Individual CLA or am covered by my company's Corporate CLA.
• I have discussed my proposed solution with code owners in the linked issue(s) and we have agreed upon the general approach.
• I have made any needed documentation changes, or noted in the linked issue(s) that documentation elsewhere needs updating.
• I have added tests, or I have ensured existing tests cover the changes
• I have followed Google's Python Style Guide and ran pylint over the affected code.