Allow tokens in context_display labels, and label-only status lines

The webform interaction context_display setting (added in #3608397) posts one Drupal message per entry, rendered as "label: value": the value comes from the entry variable, but the label is a plain, static string. There is no way to put dynamic content in the label, so a status line like "Reviewed by {a person}" or a label that varies with a process value is not possible today.

Proposal

• Run each entry label through the core token service, exactly as the message interaction already does for its body: replace against the parked instance and branch (the orchestra and orchestra_token contexts), clear unknown tokens, and keep the default escaping so a resolved value is inserted as data, not markup.
• Allow an empty variable, so a label-only entry renders just the tokenized label with no trailing ": value". A line such as "Reviewed by [orchestra:var:validator_uid:user:display-name]" then reads as a complete sentence.
• Offer the token tree picker in the settings form when the optional token module is present, again mirroring the message interaction.

Resolving a person name already works

The motivating example is showing who reviewed a request. The examples capture the completer user id in validator_uid, and the orchestra var token already chains through an entity reference: ProcessTokens::chainReference() handles var:{name}:{entity_type}:{chain}, documented as [orchestra:var:validator_uid:user:display-name]. So no name capture or new token is needed; once the label is tokenized, that chained token resolves the reviewer name directly. This issue is therefore just the label tokenization and label-only lines.

Tests

• A label containing [orchestra:var:NAME] resolves against the instance and branch.
• A label containing a chained reference such as [orchestra:var:validator_uid:user:display-name] resolves the referenced entity.
• An empty variable yields a label-only line (no ": value").
• Resolved values stay escaped (no markup injection through a variable value).