fix(go): correct buffer size for CreateUser and UpdatePermissions serialization

[atharvalade]

Which issue does this PR close?

Closes #2981

Rationale

The Go SDK's CreateUser and UpdatePermissions commands had incorrect buffer size calculations for the has_permissions flag byte, causing a 1-byte over-allocation in CreateUser (non-nil path) and an index-out-of-bounds panic in UpdatePermissions (nil path).

What changed?

Both CreateUser.MarshalBinary() and UpdatePermissions.MarshalBinary() double-counted the 1-byte has_permissions flag in the non-nil branch, while UpdatePermissions omitted it entirely from the base allocation (causing a panic when permissions were nil).

The fix accounts for the has_permissions byte exactly once: in CreateUser it is part of the base capacity 4, and in UpdatePermissions it is now added to the base length. The +1 was removed from the non-nil branch in both methods. Unit tests were added for all four cases (nil/non-nil x CreateUser/UpdatePermissions).

Local Execution

• Passed
• Pre-commit hooks ran (trailing-whitespace, trailing-newline, license-headers all pass; go test ./... all pass; gofmt clean; go vet clean)

AI Usage

1. Opus 4.6
2. Minimal AI used
3. Yes, all code can be explained

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.72%. Comparing base (27f0f11) to head (8ec4cfd).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #3097   +/-   ##
=========================================
  Coverage     72.71%   72.72%           
  Complexity      943      943           
=========================================
  Files          1117     1117           
  Lines         96285    96285           
  Branches      73485    73485           
=========================================
+ Hits          70014    70020    +6     
+ Misses        23725    23719    -6     
  Partials       2546     2546           

Components	Coverage Δ
Rust Core	73.45% <ø> (ø)
Java SDK	62.30% <ø> (ø)
C# SDK	69.40% <ø> (ø)
Python SDK	81.43% <ø> (ø)
Node SDK	91.53% <ø> (ø)
Go SDK	39.09% <100.00%> (+0.12%)	⬆️

Files with missing lines	Coverage Δ
foreign/go/internal/command/user.go	91.30% <100.00%> (+6.52%)	⬆️

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