Remove UNSAFE_TODO usages in file system utils

Replaced each UNSAFE_TODO with size-bounded container/view operations to
remove raw pointer arithmetic while preserving behavior. In
obfuscated_file_util_memory_delegate.cc, append/overwrite now use
IOBuffer::first(...) + vector::insert(...) /
span(...).subspan(...).copy_from(...) after existing offset/length
validation and resize, so both source and destination are explicitly
in-bounds. In file_system_util.cc, switched from indexed/pointer-offset
access (kSeparators[i], kXxxDir + 1) to range/string-view based
operations. In dump_file_system.cc, moved suffix handling to string
composition. Overall this keeps semantics unchanged but makes bounds
explicit and verifiable.

Bug: 40284755
Change-Id: Ic8042b3f8bae93fe6469f33b99693c698d308694
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7626019
Commit-Queue: kelsen liu <kelsenliu@chromium.org>
Reviewed-by: Ming-Ying Chung <mych@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1593744}