chore: improve coverage of cryptorand package

[jawnsy]

Check error cases in cryptorand functions, such as failures to
read random data and input parameter validation.

[codecov]

Codecov Report

Merging #377 (258c918) into main (0c005ce) will increase coverage by 0.44%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #377      +/-   ##
==========================================
+ Coverage   67.93%   68.38%   +0.44%     
==========================================
  Files         148      150       +2     
  Lines        8096     8274     +178     
  Branches       72       72              
==========================================
+ Hits         5500     5658     +158     
- Misses       2032     2051      +19     
- Partials      564      565       +1     

Flag	Coverage Δ
unittest-go-macos-latest	66.94% <ø> (+0.76%)	⬆️
unittest-go-ubuntu-latest	67.95% <ø> (+0.77%)	⬆️
unittest-go-windows-2022	66.05% <ø> (?)
unittest-js	66.10% <ø> (ø)

Impacted Files	Coverage Δ
pty/start_windows.go	51.30% <0.00%> (ø)
pty/pty_windows.go	60.31% <0.00%> (ø)
peerbroker/listen.go	84.80% <0.00%> (+2.39%)	⬆️
peer/conn.go	81.07% <0.00%> (+2.55%)	⬆️
pty/ptytest/ptytest.go	90.74% <0.00%> (+5.55%)	⬆️
cryptorand/strings.go	90.32% <0.00%> (+9.67%)	⬆️
cryptorand/numbers.go	84.07% <0.00%> (+37.16%)	⬆️

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[jawnsy]

[sc-21247]