[ java ] Add code samples for missing content

[rachfop]

The following sections need code samples.

Provide a code sample per section by using the comment section to provide a code snippet.
Mark the checkbox complete as sample code is added.

If the language does not support a feature, write “NA” or link/code to a related concept: for an example, see Go’s Remove Search Attribute.

Testing

• Assert In Workflow
• Automatic Method
• Cancel An Activity
• Listen To Heartbeats
• Manual Method
• Mock Activities
• Replay
• Run An Activity
• Setting Up
• Skip Time In Activities
• Skip Time
• Test Activities
• Test Frameworks
• Test Workflows
• Workflow Context

Observability

• Search Attributes
• Visibility

Features

• Environment Variables
• Mutable Side Effects

[rachfop]

Closing as it is tracked internally now.