*: fix QUIC false error

[KaloyanTanev]

No need to error on closing the write if the stream was already canceled.
Change was provoked by this failing unit test:
https://github.com/ObolNetwork/charon/actions/runs/28664948834/job/85014276978?pr=4575

category: misc
ticket: none

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.29%. Comparing base (087db8b) to head (b486119).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
p2p/sender.go	25.00%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4585   +/-   ##
=======================================
  Coverage   57.29%   57.29%           
=======================================
  Files         245      245           
  Lines       33537    33559   +22     
=======================================
+ Hits        19215    19228   +13     
- Misses      11882    11892   +10     
+ Partials     2440     2439    -1     

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Copilot]

Pull request overview

This PR prevents benign QUIC “canceled stream” errors from failing request/response flows by treating that specific close error as non-fatal (especially when the peer has already reset the send direction), while still surfacing other close failures.

Changes:

• Treat “canceled stream” errors from CloseWrite() in SendReceive as benign (debug log + continue).
• Replace repeated string checks with a shared helper isCanceledStreamErr.
• Add unit tests covering isCanceledStreamErr behavior (including wrapped errors).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
p2p/sender.go	Ignores benign canceled-stream errors on close paths (incl. CloseWrite) and centralizes detection logic in a helper.
p2p/sender_internal_test.go	Adds focused tests to lock in the helper’s behavior for nil/other/canceled/wrapped errors.

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