Update param-forward-tsn-supported to RFC 9260

[philipch07]

Description

• Adds a reference to RFC 9260 since it previously didn't refer to anything
• Adds a guard for if raw is empty in unmarshal().
• Adds tests for the guard

Reference issue

Resolves #422.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.85%. Comparing base (a69c6a2) to head (f8adf77).
⚠️ Report is 37 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #423   +/-   ##
=======================================
  Coverage   83.84%   83.85%           
=======================================
  Files          51       51           
  Lines        3448     3450    +2     
=======================================
+ Hits         2891     2893    +2     
  Misses        417      417           
  Partials      140      140           

Flag	Coverage Δ
go	83.85% <100.00%> (+<0.01%)	⬆️

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[Copilot]

Pull Request Overview

This PR adds validation for the Forward TSN Supported parameter to ensure it conforms to RFC 9260 requirements. The parameter must be header-only (length=4) with no payload data.

Key Changes:

• Added validation in unmarshal to reject parameters with non-empty payloads
• Introduced a new error type ErrForwardTSNSupportedParamInvalidLength for invalid length errors
• Added comprehensive test coverage for both invalid unmarshal and marshal edge cases

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
param_forward_tsn_supported.go	Added error constant and validation logic to enforce header-only parameter format
param_forward_tsn_supported_test.go	Added two new test cases covering invalid length unmarshal and marshal behavior with pre-filled raw data

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