fix(cli): point to Gemini Enterprise registration docs after agent_engine deploy

[chelsealong]

Fixes #6633

Problem

adk deploy agent_engine has no way to push directly to Gemini
Enterprise, and after a successful deploy there was no indication of
how to make the agent available there. The issue's proposed
alternative was for adk deploy to at least tell users how to do it.

Fix

After a successful adk deploy agent_engine, print a pointer to the
Gemini Enterprise registration docs
(https://docs.cloud.google.com/gemini/enterprise/docs/register-and-manage-an-adk-agent),
reusing the same doc link already referenced by the
--gemini_enterprise_app_name flag on adk api_server.

This is a minimal, additive change: no new deploy target, no new
auth/credential handling, just a helpful hint printed alongside the
existing "Deployed to Agent Platform" / playground URL messages in
src/google/adk/cli/cli_deploy.py.

Testing plan

Added test_print_gemini_enterprise_hint in
tests/unittests/cli/utils/test_cli_deploy.py, mirroring the existing
test_print_agent_engine_url test.

Verified the test fails without the fix (checked out the pre-fix
version of cli_deploy.py and reran):

$ python -m pytest tests/unittests/cli/utils/test_cli_deploy.py -k test_print_gemini_enterprise_hint -q
FAILED tests/unittests/cli/utils/test_cli_deploy.py::test_print_gemini_enterprise_hint - AttributeError: module 'src.google.adk.cli.cli_deploy' has no attribute '_print_gemini_enterprise_hint'
1 failed, 45 deselected, 2 warnings in 0.33s

With the fix applied, the full deploy test file passes:

$ python -m pytest tests/unittests/cli/utils/test_cli_deploy.py -q
46 passed, 25 warnings in 1.66s

Also ran pyink --check and isort --check on both changed files
with no diffs.

AI assistance disclosure

This change was developed with the help of an AI coding agent (Claude
Code), including drafting the fix, the test, and this description. All
changes were reviewed and verified locally before submission.