fix: #3583083 State the entity input violation's reason only and let Drush add its own advice

Closes #3583083 (closed)

The "must be an entity" violation told every invoker without InvokerCapability::EntitiesAsHandles that it cannot pass entities and should look for a tool that takes an entity type and ID (#3583046 (closed)). That capability is the switch for Tool API's own handle subscriber, not a statement about what an invoker can deliver: input transforms are an open event, so any module can turn what an invoker sent into an entity, and an in-process invoker such as eca_tool's hands tools real entity objects. Validation cannot know the answer in either polarity, so it stops guessing.

• TypedInputsTrait states the reason only, This input must be an entity., for every invoker, with the input on the property path as for every other violation (#3583085 (closed)). The capability check and the invoker-named hint are gone.
• The violation carries a code, TypedInputsInterface::NOT_AN_ENTITY_ERROR, so an invoker can add advice of its own by matching the code rather than the message or the input's type. The code survives the rebuilds that put a list item's or map property's position on the path.
• drush tool:run, which passes only strings and JSON, adds its own line after the violations when one carries that code: drush tool:run cannot pass entities; use a tool that takes an entity type and ID instead. A missing required entity input, or a constraint failure on a value something did turn into an entity, gets no such line.
• .agents/skills/use-drupal-tools/SKILL.md shows the current Drush output for an entity input.
• No new InvokerCapability case: the earlier EntitiesAsObjects approach on this branch is reverted, so Invoker, tool.api.php and the entity handles docs are unchanged.

Tests: EntityInputTypeCheckTest::testEntityViolationIsTheReasonForEveryInvoker (no invoker, an invoker with no capabilities, a handle-capable invoker: the same message and code, and the code on a list item's violation) and ToolRunCommandTest::testEntityViolationNamesTheWorkaround (the reason on the entity: path plus the command's advice, and no advice for a violation on a non-entity input).

Release note: The "This input must be an entity." violation no longer names the invoker or suggests another tool; it carries the code TypedInputsInterface::NOT_AN_ENTITY_ERROR. drush tool:run adds that advice itself for a value that is not an entity.