enh: improve envelope conversion to Uint8List in FileSystemTransport

[buenaflor]

💡 Motivation and Context

Performance improvement by avoiding a copy

Benchmarks tested on release mode on Android

I/flutter ( 4261): Envelope Builder Benchmark
I/flutter ( 4261): ==========================
I/flutter ( 4261): Comparing legacy List<int> vs BytesBuilder approaches
I/flutter ( 4261): Envelope size: 1 KB
I/flutter ( 4261): ----------------------------------------
I/flutter ( 4261): Running 1000 iterations...
I/flutter ( 4261): Legacy approach (List<int> + addAll):
I/flutter ( 4261):   Average: 6.2 μs
I/flutter ( 4261):   Min: 5.0 μs
I/flutter ( 4261):   Max: 488.0 μs
I/flutter ( 4261): New approach (BytesBuilder):
I/flutter ( 4261):   Average: 2.1 μs
I/flutter ( 4261):   Min: 2.0 μs
I/flutter ( 4261):   Max: 68.0 μs
I/flutter ( 4261): Performance improvement: 65.7% (2.91x faster)
I/flutter ( 4261): 
I/flutter ( 4261): Envelope size: 10 KB
I/flutter ( 4261): ----------------------------------------
I/flutter ( 4261): Running 1000 iterations...
I/flutter ( 4261): Legacy approach (List<int> + addAll):
I/flutter ( 4261):   Average: 62.3 μs
I/flutter ( 4261):   Min: 54.0 μs
I/flutter ( 4261):   Max: 455.0 μs
I/flutter ( 4261): New approach (BytesBuilder):
I/flutter ( 4261):   Average: 28.6 μs
I/flutter ( 4261):   Min: 27.0 μs
I/flutter ( 4261):   Max: 247.0 μs
I/flutter ( 4261): Performance improvement: 54.1% (2.18x faster)
I/flutter ( 4261): 
I/flutter ( 4261): Envelope size: 100 KB
I/flutter ( 4261): ----------------------------------------
I/flutter ( 4261): Running 500 iterations...
I/flutter ( 4261): Legacy approach (List<int> + addAll):
I/flutter ( 4261):   Average: 1.31 ms
I/flutter ( 4261):   Min: 1.01 ms
I/flutter ( 4261):   Max: 2.88 ms
I/flutter ( 4261): New approach (BytesBuilder):
I/flutter ( 4261):   Average: 287.4 μs
I/flutter ( 4261):   Min: 275.0 μs
I/flutter ( 4261):   Max: 667.0 μs
I/flutter ( 4261): Performance improvement: 78.1% (4.56x faster)
I/flutter ( 4261): 
I/flutter ( 4261): Envelope size: 1 MB
I/flutter ( 4261): ----------------------------------------
I/flutter ( 4261): Running 200 iterations...
I/flutter ( 4261): Legacy approach (List<int> + addAll):
I/flutter ( 4261):   Average: 18.78 ms
I/flutter ( 4261):   Min: 15.30 ms
I/flutter ( 4261):   Max: 22.53 ms
I/flutter ( 4261): New approach (BytesBuilder):
I/flutter ( 4261):   Average: 3.42 ms
I/flutter ( 4261):   Min: 2.76 ms
I/flutter ( 4261):   Max: 5.10 ms
I/flutter ( 4261): Performance improvement: 81.8% (5.50x faster)
I/flutter ( 4261): 
I/flutter ( 4261): Envelope size: 5 MB
I/flutter ( 4261): ----------------------------------------
I/flutter ( 4261): Running 50 iterations...
I/flutter ( 4261): Legacy approach (List<int> + addAll):
I/flutter ( 4261):   Average: 74.45 ms
I/flutter ( 4261):   Min: 68.71 ms
I/flutter ( 4261):   Max: 85.26 ms
I/flutter ( 4261): New approach (BytesBuilder):
I/flutter ( 4261):   Average: 17.92 ms
I/flutter ( 4261):   Min: 15.56 ms
I/flutter ( 4261):   Max: 21.43 ms
I/flutter ( 4261): Performance improvement: 75.9% (4.16x faster)

💚 How did you test it?

Manual + existing tests

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• No new PII added or SDK only sends newly added PII if sendDefaultPii is enabled
• I updated the docs if needed
• All tests passing
• No breaking changes

🔮 Next steps

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.90%. Comparing base (6b69699) to head (e66fb57).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3147      +/-   ##
==========================================
+ Coverage   87.66%   89.90%   +2.23%     
==========================================
  Files         291       96     -195     
  Lines        9884     3427    -6457     
==========================================
- Hits         8665     3081    -5584     
+ Misses       1219      346     -873     

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[denrase]

Very nice! Do we need to keep the benchmark test around?

[buenaflor]

@denrase I'd just keep it there