fix(mtproxy): decrypt only bytes actually read in Obfuscated2.Read

[lavr]

XORKeyStream(b, b) advanced the AES-CTR stream by len(b) instead of n, desyncing decryption when FakeTLS returns partial reads (one TLS record at a time). This corrupted all large MTProxy responses spanning multiple TLS records.

Fixes #1139

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.46%. Comparing base (36ad1b1) to head (17a06a5).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1694      +/-   ##
==========================================
- Coverage   68.51%   68.46%   -0.06%     
==========================================
  Files         452      452              
  Lines       18910    18910              
==========================================
- Hits        12957    12947      -10     
- Misses       4987     4995       +8     
- Partials      966      968       +2     

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[ernado]

This is great, thank you!
For some reason TestExternalE2EMTProxy/Tls now fails, need to investigate.

Upd: the reason was in mtg version