fix(tools): support OpenAPI basic auth

[aashnakunk]

Fixes #5493

Summary

• Add HTTP Basic auth header generation for OpenAPI tools
• Add unit coverage for Basic auth credentials

Testing

• .venv/bin/pytest tests/unittests/tools/openapi_tool/auth/test_auth_helper.py

Result:

• 36 passed, 1 warning in 9.32s

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[adk-bot]

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