test: add exhaustive parity and integration tests for Azure/GCP v2

- test_azure_v2_parity.py: 157 parametrized tests covering
  constructor, context manager, all chat/ocr params, sync/async parity
- test_gcp_v2_parity.py: 188 parametrized tests covering
  constructor, context manager, all chat/fim params, sync/async parity
- test_azure_integration.py: credential-gated integration tests
  for Azure chat (sync/async/streaming/tools/JSON)
- test_gcp_integration.py: credential-gated integration tests
  for GCP chat + FIM (sync/async/streaming/tools)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Nelson-PROIA