Optimize HandleMalformedStrings middleware for CPU and memory

What does this MR do and why?

Previously HandleMalformedStrings always duplicated a string and checked whether it was valid UTF-8. However, this leads to unnecessary memory allocations and CPU usage.

We can significantly optimize this by only duplicating the string when we need to force the encoding to UTF-8.

How to set up and validate locally

The existing tests all pass, but you can see the performance improvements with:

benchmark.rb

### String Checks (Higher is better) ###

ruby 3.3.9 (2025-07-24 revision f5c772fc7c) [arm64-darwin24]
Warming up --------------------------------------
  Old - clean string   818.455k i/100ms
  New - clean string     1.257M i/100ms
Calculating -------------------------------------
  Old - clean string      8.203M (± 1.2%) i/s  (121.90 ns/i) -     41.741M in   5.088950s
  New - clean string     12.234M (±10.1%) i/s   (81.74 ns/i) -     60.337M in   5.008126s

Comparison:
  New - clean string: 12234428.9 i/s
  Old - clean string:  8203465.1 i/s - 1.49x  slower


### Long String (10k chars) ###

ruby 3.3.9 (2025-07-24 revision f5c772fc7c) [arm64-darwin24]
Warming up --------------------------------------
    Old - long clean    26.319k i/100ms
    New - long clean    72.311k i/100ms
Calculating -------------------------------------
    Old - long clean    254.537k (±13.4%) i/s    (3.93 μs/i) -      1.237M in   5.033226s
    New - long clean    723.368k (±20.7%) i/s    (1.38 μs/i) -      3.471M in   5.042592s

Comparison:
    New - long clean:   723368.0 i/s
    Old - long clean:   254536.6 i/s - 2.84x  slower


### Null Byte Detection ###

ruby 3.3.9 (2025-07-24 revision f5c772fc7c) [arm64-darwin24]
Warming up --------------------------------------
Old - null in middle   820.819k i/100ms
New - null in middle     1.846M i/100ms
   Old - null at end    19.003k i/100ms
   New - null at end    41.054k i/100ms
Calculating -------------------------------------
Old - null in middle      8.190M (± 1.1%) i/s  (122.11 ns/i) -     41.041M in   5.011969s
New - null in middle     18.062M (±10.1%) i/s   (55.37 ns/i) -     88.602M in   5.000721s
   Old - null at end    196.989k (± 6.7%) i/s    (5.08 μs/i) -    988.156k in   5.042065s
   New - null at end    412.956k (±24.3%) i/s    (2.42 μs/i) -      1.888M in   5.076560s

Comparison:
New - null in middle: 18061641.2 i/s
Old - null in middle:  8189596.6 i/s - 2.21x  slower
   New - null at end:   412955.6 i/s - 43.74x  slower
   Old - null at end:   196989.4 i/s - 91.69x  slower

MR acceptance checklist

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