Support HTTP Basic auth for OpenAPI tools

[Oppong08]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: Support HTTP Basic authentication for OpenAPI tools #5492
• Related: N/A

2. Or, if no issue exists, describe the change:

N/A

Problem:
OpenAPI tools currently do not support HTTP Basic authentication. When an OpenAPI auth credential contains HTTP username/password credentials, ADK raises NotImplementedError("Basic Authentication is not supported.") instead of generating a standard Basic auth Authorization header.

Solution:
Add HTTP Basic auth support in credential_to_param by mirroring the existing bearer-token handling pattern. When HTTP credentials contain a username and/or password, the helper now encodes username:password with base64 and returns an internal Authorization header in the format Basic <base64(username:password)>. This keeps the change small, localized, and consistent with the current auth helper design.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Passed locally:

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

Manual End-to-End (E2E) Tests:

Manual verification performed by exercising the auth helper path locally and confirming that HTTP Basic auth credentials now produce an internal Authorization header parameter and kwargs instead of raising NotImplementedError.

Relevant behavior verified:

• username + password generates Authorization: Basic <base64(username:password)>
• existing non-Basic auth paths remain unchanged

Additional commands run:

• pre-commit run --all-files

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

This change is intentionally minimal:

• adds base64 import
• replaces the unsupported HTTP Basic auth branch in src/google/adk/tools/openapi_tool/auth/auth_helpers.py
• updates tests/unittests/tools/openapi_tool/auth/test_auth_helper.py

The implementation preserves existing behavior for non-Basic auth paths.

Add any other context or screenshots about the feature request here.

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