fix(harness): inherit parent execution configs in declared subagents (#2252)

## Description

**Background.** `HarnessAgentBuilderSupport` has two subagent factories
that are meant to behave consistently: `buildGeneralPurposeFactory` and
`buildDeclaredFactory`. The general-purpose factory captures the parent
agent's `modelExecutionConfig` / `toolExecutionConfig` and applies them
to the child, but `buildDeclaredFactory` never did.