Read current task executions by exact region and try

[ashb]

A loop pass or a mapped expansion reuses a task id and map index, so reading
a task instance, its XCom, logs, links, HITL detail or state by (task_id,
map_index) no longer names one execution. The public readers take region_id and
region_index and resolve them against the live producers. An index without a
region is rejected because an index means nothing without the region it indexes.
Responses return the region coordinates and a map_index derived from the pinned
definition, so mapped tasks keep presenting their slot as before while a task
inside a loop presents -1. Filters and sorts use the same derived value so
paging and cursors agree with what the responses show.

Retired attempts stay in task_instance under their own UUID, so try details,
logs and external log links select one by try_number plus the same coordinates.
The external log URL used to ignore the try number, which became ambiguous once
several rows share a coordinate. Operator extra links read their XCom from the
selected region and pinned definition, since the coordinates alone no longer say
which execution produced it.

The run-level execution listing returns the region ancestry beside each live
task instance because its coordinates cannot be interpreted without it. The
Gantt payload carries the attempt id and region coordinates so a bar identifies
one attempt.

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