Relocate ContextMessages to the base module, render in-place context inline, and show variable scope in the instance view

Follow-up cleanup of the context_display feature (#3608397, #3608435), driven by a layering smell: the context-message handling had crept across submodules.

Relocate and clean up ContextMessages

• The service depends only on the engine, the messenger and the token service (no webform coupling), so it moves from orchestra_interaction_webform to the base orchestra_interaction module, where any submodule can reuse it. Its docblocks no longer name a specific consumer.
• A step rendered in place (a review inside a task, a dispatcher status page) now renders its context inline through a new build() method (a status_messages render array), as part of the interaction output. It appears with the step and is gone when the run advances, so there is nothing to retract and no other module is involved.
• display()/retract() (the messenger path) remain for the one case a render array cannot reach: a step that redirects the actor to a separate surface. The consumer that posts there (the webform handler) is the one that retracts, on completion.
• This removes the cross-submodule reference the task form had grown (it no longer touches context messages at all).

Instance view: variable scope

• The Variables table on the process instance page gains a Scope column: "Instance" for instance-wide variables, or the node and token id ("n_validate (#1836)") for a token-scoped one, so two rows of the same name written on the same node in different loop rounds are told apart.

Examples

• The validate step now shows the previous validation comment on a re-review (empty, so hidden, on the first pass).
• The gratuitous "Reviewed by {name}" line on the process step is dropped (the label-token feature it demonstrated stays in code and tests).

Tests: ContextMessagesTest moves to the base module and covers build() (render array) alongside the messenger display()/retract(); the request-validation Functional test covers the re-review comment and the inline render; no orchestra_ui test regressed.