test: add failing coverage for Claude OAuth auth regression

[atulsalvekar]

Summary

• add failing unit regression coverage for Anthropic OAuth auth JSON with is_error: true
• add failing workflow-level E2E coverage for fallback and no-fallback paths
• document the current red-state behavior so the implementation fix can target the active regression

Verification

• pytest -q tests/test_agentic_common.py -k 'anthropic_oauth_error or anthropic_success_json_still_succeeds_with_is_error_false'
• python -m py_compile tests/test_e2e_issue_743_anthropic_oauth_error.py
• pytest -q tests/test_e2e_issue_743_anthropic_oauth_error.py

Current Red State

• unit regression: 3 failures, 1 pass
• E2E regression: 2 failures

Fixes #743

PDD Fix Command

pdd fix https://github.com/promptdriven/pdd/issues/743