fix: VariableTruncator._truncate_array under-reports truncated flag

[ifer47]

Summary

• Fix _truncate_array size-limit break not setting truncated = True before exiting the loop, causing dropped tail elements to go unreported
• Fix _truncate_array overwriting the truncated flag on each iteration instead of accumulating it (truncated = truncated or part_result.truncated), so a later non-truncated element could erase an earlier truncation signal
• Add two targeted unit tests reproducing the exact scenarios from the issue

Context

Closes #37192

The _truncate_object and truncate_variable_mapping sibling methods already accumulate the flag correctly. This PR brings _truncate_array in line with them.

Bug 1 — size-limit break: When used_size > target_size, the loop breaks but leaves truncated = False, so dropped array elements are not reported:

t = VariableTruncator(array_element_limit=20, max_size_bytes=1000)
r = t._truncate_array([10, 20, 30, 40, 50], 12)
# Before fix: r.truncated == False (wrong — element 50 was dropped)
# After fix:  r.truncated == True

Bug 2 — flag overwrite: truncated = part_result.truncated replaces the flag on each iteration instead of OR-accumulating:

t2 = VariableTruncator(array_element_limit=20, max_size_bytes=1000, string_length_limit=10)
r2 = t2._truncate_array(["x" * 60, "a"], 60)
# Before fix: r2.truncated == False (wrong — "x"*60 was truncated)
# After fix:  r2.truncated == True

Test plan

• New test test_array_size_limit_break_sets_truncated_flag verifies bug 1
• New test test_array_truncated_flag_accumulates_across_elements verifies bug 2
• Existing TestArrayTruncation tests continue to pass (no behavior change for non-buggy paths)

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