Pickle serialisation process & LazyLoad tools documentation

[tommyhutcheson]

Is your feature request related to a problem? Please describe.

When deploying agents to Vertex AI using ADK's agent_engines.create(), environment variables specified in
AdkApp.env_vars are not available when toolsets are initialised during the pickle serialisation process. This causes
pickled toolsets to retain configuration values from the deployment environment rather than reading from the runtime environment.

Impact
This behaviour breaks environment specific configuration patterns and forces developers to implement complex workarounds using lazy loading and custom pickle serialisation to defer toolset creation until runtime.

google/adk-python#3208

Describe the solution you'd like

Clear documentation about lazy loading toolsets and agent engine pickle serialisation proces.

[tommyhutcheson]

PR raised -> #852