fix(trace): return removed row from pop()

[Dev-X25874]

Description

Both Table.pop() in weave/trace/table.py and WeaveTable.pop() in
weave/trace/vals.py were declared -> None and discarded the return
value of the underlying list.pop() call. Python's list.pop() removes
and returns the element at the given index; these implementations
silently broke that contract, meaning any caller doing:

row = my_table.pop(0)

would always receive None instead of the removed dict.

Fix: capture and return the result of the inner list.pop() in both
methods, and update their return type annotations from -> None to
-> dict.

Testing

Verified by code inspection that both call sites previously discarded
the list.pop() return value. No existing tests cover Table.pop() or
WeaveTable.pop() (confirmed via grep), so no tests were broken. Manual
smoke-test of the corrected behaviour:

t = Table([{"a": 1}, {"b": 2}])
row = t.pop(0)
assert row == {"a": 1}   # was None before the fix
assert len(t) == 1

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