Update ack_timer to RFC 9260

[philipch07]

Description

• Updates references in ack_timer to RFC 9260
• Adds a clamp for the max delay according to RFC 9260

Note that this doesn't currently allow for the delay to be configurable, but the clamp is there in case the default value changes to ensure correctness.

Reference issue

Resolves #428.

[codecov]

Codecov Report

❌ Patch coverage is 69.23077% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.85%. Comparing base (a69c6a2) to head (086aa13).

Files with missing lines	Patch %	Lines
ack_timer.go	69.23%	2 Missing and 2 partials ⚠️

❌ Your patch check has failed because the patch coverage (69.23%) is below the target coverage (70.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #429      +/-   ##
==========================================
+ Coverage   83.84%   83.85%   +0.01%     
==========================================
  Files          51       51              
  Lines        3448     3457       +9     
==========================================
+ Hits         2891     2899       +8     
  Misses        417      417              
- Partials      140      141       +1     

Flag	Coverage Δ
go	83.85% <69.23%> (+0.01%)	⬆️

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[Copilot]

Pull Request Overview

This PR refactors the ackTimer to support configurable SACK (Selective Acknowledgement) delays per RFC 9260 requirements. The changes improve RFC compliance, fix spelling errors, enhance defensive programming around timer state management, and update documentation to reflect the correct RFC version.

• Added configurable SACK delay with proper validation bounds (0, 500ms]
• Improved defensive checks on pending counter to prevent underflow
• Updated documentation and corrected spelling mistakes

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