fix(cli): use explicit re-export in __init__.py generated by adk create

[chelsealong]

Fixes #6648

Problem

adk create generates an __init__.py for code-based agents containing:

from . import agent

This import is intentional — it makes the agent module (and its
root_agent) discoverable — but Ruff's F401 rule flags it as an
unused import in downstream projects, since the intent isn't expressed
explicitly.

Fix

Generate an explicit re-export instead, using the redundant-alias form
that Ruff recognizes as intentional:

from . import agent as agent

This preserves the existing runtime behavior while making the
re-export explicit to linters, so newly generated agent projects pass
common lint configs without suppressions.

Testing plan

Updated test_generate_files_with_api_key in
tests/unittests/cli/utils/test_cli_create.py to assert the exact
generated __init__.py content instead of just its existence.

Verified the test fails without the fix (reverted the one-line source
change while keeping the updated test):

$ python -m pytest tests/unittests/cli/utils/test_cli_create.py -k test_generate_files_with_api_key -v
...
E     AssertionError: assert 'from . import agent\n' == 'from . impor...nt as agent\n'
E       - from . import agent as agent
E       ?               ---------
E       + from . import agent
FAILED tests/unittests/cli/utils/test_cli_create.py::test_generate_files_with_api_key
1 failed, 29 deselected in 1.21s

With the fix restored:

$ python -m pytest tests/unittests/cli/utils/test_cli_create.py -v
...
30 passed in 7.52s

Full tests/unittests/cli suite also passes:

$ python -m pytest tests/unittests/cli -q
770 passed, 5 skipped, 4 xfailed, 179 warnings in 46.03s

pre-commit run --files src/google/adk/cli/cli_create.py tests/unittests/cli/utils/test_cli_create.py
passes cleanly (ruff, isort, pyink, addlicense, ADK compliance checks).

AI assistance disclosure

This change was drafted with AI assistance (Claude Code) and reviewed
before submission.