Issue #3606290: ProcessInstance needs a non-null label to render as a webform source entity

Follow-up to #3606290. orchestra_interaction_webform binds a webform submission to its workflow instance as the source entity. Rendering that submission links the source entity by its label, and ProcessInstance had no label (no label key, no override), so label() returned NULL and Html::escape(NULL) threw when the link was built (seen on viewing or going back to a submission).

This adds a stable ProcessInstance label (" #") so an instance is safe to render wherever it is linked (a webform source entity, an entity reference, the canonical title), plus a functional test (SourceEntityRenderTest) that views an instance-bound submission and asserts it renders with the instance label shown.

The fix landed on the previous branch after !220 was assembled, so it did not make it into the merge; this re-submits it on its own.