fix: VariableTruncator._truncate_array under-reports truncated flag

[functionkiller]

Summary

Fixes two bugs in VariableTruncator._truncate_array that caused the truncated flag to be under-reported:

1. Size-limit break path: When used_size > target_size, the loop breaks without setting truncated = True, so dropped tail elements are not reflected in the flag.
2. Overwrite instead of accumulation: truncated = part_result.truncated overwrites the flag on each iteration. If an earlier element was truncated but a later element fits, the flag is incorrectly reset to False.

Both sibling methods (truncate_variable_mapping and _truncate_object) correctly accumulate the flag with |= or or.

Changes

• Added truncated = True before the size-limit break
• Changed truncated = part_result.truncated to truncated = truncated or part_result.truncated

Test plan

• Case 1: size-limit break — truncated=True when tail elements are dropped
• Case 2: flag accumulation — truncated flag preserved when later element fits
• Case 3: no truncation — truncated=False unchanged
• Case 4: element limit — truncated=True unchanged

Fixes #37192