fix(encoding): encodeVarint() throws when value or buffer overflows

[truffle-dev]

Fixes #7147.

encodeVarint() silently truncated when the input exceeded uint64 with the default 10-byte buffer. The loop bound i <= Math.min(buf.length, MaxVarintLen64) allowed an out-of-bounds write that Uint8Array silently dropped, and the final num < MSBN branch returned a tuple whose array length and reported offset disagreed:

encodeVarint(0x1234567891234567891n);
// [Uint8Array(10) [...10 bytes...], 11]   <- offset says 11 but array is 10

The fix adds an explicit num > MaxUint64 check before the loop and tightens the loop bound to i < buf.length. The trailing throw now reports the specific failure mode (buffer too small), since uint64 overflow is caught upfront. Two regression tests cover both branches: a uint64-overflow input on the default buffer, and a valid uint64 value on a buffer that's too small.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.61%. Comparing base (95a1e2e) to head (9358a8b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7149      +/-   ##
==========================================
- Coverage   94.61%   94.61%   -0.01%     
==========================================
  Files         634      634              
  Lines       51843    51847       +4     
  Branches     9346     9348       +2     
==========================================
+ Hits        49050    49053       +3     
  Misses       2218     2218              
- Partials      575      576       +1     

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