fix: use crypto/rand for the retry jitter

[samber]

The Bearer scan in the Lint workflow fails on main since the retry jitter was added: math/rand triggers the weak PRNG rule (CWE-327) at dispatcher.go:153.

• draw the jitter from crypto/rand instead of suppressing the finding
• fall back to the plain backoff if no randomness is available

golangci-lint (0 issues) and bearer scan (no failures) pass locally.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.37%. Comparing base (50c86d3) to head (88f9a2b).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
dispatcher.go	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
- Coverage   92.39%   91.37%   -1.02%     
==========================================
  Files           3        3              
  Lines         171      174       +3     
==========================================
+ Hits          158      159       +1     
- Misses          7        8       +1     
- Partials        6        7       +1     

Flag	Coverage Δ
unittests	91.37% <50.00%> (-1.02%)	⬇️

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