docs(integrations): use Agent Platform client in Express Mode example

[Zen-cronic]

What

Use agentplatform.Client in the Express Mode Agent Runtime example instead of the deprecated
vertexai.Client entry point.

Why

Running the documented client construction with google-cloud-aiplatform==1.165.1 emits:

FutureWarning: The vertexai.Client class is deprecated. Please use agentplatform.Client instead.

The page currently directs new Express Mode users through that deprecated constructor.

How

• Import agentplatform instead of vertexai.
• Construct agentplatform.Client with the existing Express Mode API key.
• Retain client.agent_engines.create(...), which is the runtime collection exposed by the
  released 1.165.1 SDK.

Test

With google-cloud-aiplatform[agent_engines]==1.165.1:

vertexai.Client warnings: [FutureWarning: ... use agentplatform.Client instead.]
agentplatform.Client warnings: []
agentplatform.Client.agent_engines: AgentEngines
agentplatform.Client.agent_engines.create: present

The check constructs the clients without sending a request. I also confirmed the edited snippet is
valid Python. Runtime creation itself still requires an Express Mode API key.

git apply --check: passed
git diff --check: passed
mkdocs build --strict: passed (230 pages indexed)

Scope

This changes only the deprecated client entry point in the Express Mode creation example. It does
not migrate other pages or adopt the unreleased client.runtimes rename.

Related

PR #2019 updates installation extras and a link later in this page. It does not change the client
constructor, so this is a separate correction.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.