Chain [orchestra:var:NAME] into a referenced entity's tokens (e.g. [orchestra:var:validator_uid:user:display-name])

Follow-up to #3607623 (nested variable paths).

[orchestra:var:NAME] resolves a process variable to its stored value. Variables often hold an entity reference id: a user id (a captured completer such as validator_uid, or a submitter), a node id, a booking id. Today the only way to show a field of the referenced entity (for example the validator's display name in a notification) is to snapshot each field into its own variable, which does not scale (one config/variable per field you want).

Proposal: let a var: token chain into a target entity's own tokens, with the entity type named in the token:

[orchestra:var:validator_uid:user:display-name]   -> the user's name
[orchestra:var:validator_uid:user:mail]           -> the user's email
[orchestra:var:some_node_ref:node:title]          -> a referenced node's title

Mechanism (in ProcessTokens): for a token of the form var:{name}:{entity_type}:{rest}, if {entity_type} is a registered entity type, load that entity by the variable's scalar value and hand {rest} to the core token generator for that type (Token::generate()). So it reuses every existing [user:*], [node:*] token for free, and generalizes to any entity-reference variable.

The bare [orchestra:var:NAME] keeps returning the raw value as today; only the chained form loads an entity. The type is explicit in the token (never guessed from the value), and an unresolvable reference clears to empty.