fix(flows): stop Agent import loading authlib, which fails test_import_loading

[vishal-bulbule]

Link to Issue or Description of Change

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

• N/A

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

Problem:

Since 665ec98 ("refactor: extract tool-resolution steps into tool_request_processors"),
tests/unittests/test_import_loading.py::test_entry_point_loads_only_allowlisted_packages
fails on main, and all five Unit Tests jobs (Python 3.10 to 3.14) fail on every CI run and on open PRs
(latest main run: https://github.com/google/adk-python/actions/runs/35460464450):

FAILED tests/unittests/test_import_loading.py::test_entry_point_loads_only_allowlisted_packages[agent]
AssertionError: 'from google.adk.agents import Agent' now loads authlib, charset_normalizer,
cryptography, joserfc, requests, urllib3, which every ADK process would pay for at startup.

The new flows/llm_flows/tools/_toolset_auth.py imports
TOOLSET_AUTH_CREDENTIAL_ID_PREFIX from auth.auth_preprocessor at module level.
auth_preprocessor imports AuthHandler, which imports the OAuth2 credential exchanger and
authlib. Before the refactor, base_llm_flow.py defined the constant itself, so nothing
extra was loaded.

Solution:

Import the constant inside resolve_toolset_auth, where it is used, as the module already
does for CredentialManager and AuthHandler. One file, one import moved.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change. (N/A: covered by the existing
  test_import_loading.py, which fails on main and passes with this change)
• All unit tests pass locally.

$ pytest tests/unittests/test_import_loading.py
main (d57c84f1):  1 failed, 14 passed
this change:      15 passed

$ pytest tests/unittests/flows tests/unittests/auth -n auto
1092 passed

$ pytest tests/unittests -n auto
main (d57c84f1):  1 failed, 15291 passed
this change:      15288 passed, 0 failed

Bisected over the commits since f33d492: the import test passes up to 6605031 and fails from
665ec98 onward.

mypy: no issues in flows/llm_flows/tools/_toolset_auth.py. pre-commit passes.

Manual End-to-End (E2E) Tests:

N/A: import-time change with no behavior change. The toolset auth path still resolves the prefix
(covered by the flows and auth tests above).

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. (N/A, one import moved)
• I have added tests that prove my fix is effective or that my feature works. (existing test)
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end. (N/A, see above)
• Any dependent changes have been merged and published in downstream modules. (N/A)