[Gap]: Document default step outputs for composite tools

[jerm-dro]

What needs documentation?

Composite tools now support configuring default return values for steps that may be skipped. This is important when:

• A step has failureMode: "continue" and an error occurs
• A step has a condition that evaluates to false

Without defaults, downstream steps that reference the skipped step's output would receive empty strings, causing unexpected behavior.

Key features that need documentation:

• The defaultResults field in composite tool step configuration
• When and why to use defaultResults
• Validation behavior: an error is produced if a step may be skipped, its outputs are referenced downstream, and no defaults are configured
• Examples showing usage with conditional steps and failureMode=continue

Context and references

Implementation issue: stacklok/toolhive#2989

Use case

As a platform engineer configuring composite tools, I want to be able to configure default step outputs in the event a step is skipped (e.g. conditional, ignored error), so that my composite tools continue to behave as expected under all possible branches.

Additional context

Example configuration:

step:
  ...
  defaultResults:
    "some_returned_value_name": "default value for downstream"