fix(eventsource): validate Last-Event-ID on reconnect

[mcollina]

UTF-8 encode EventSource last event IDs before adding them to the internal request header list. Clear stale values and omit IDs that cannot be transported as HTTP header values, preventing repeated local reconnect failures while preserving valid Unicode IDs.

Refs: GHSA-q9qx-47rg-rhc7

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.47%. Comparing base (c32ea13) to head (195facc).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5746   +/-   ##
=======================================
  Coverage   93.46%   93.47%           
=======================================
  Files         110      110           
  Lines       38934    38946   +12     
=======================================
+ Hits        36389    36403   +14     
+ Misses       2545     2543    -2     

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