feat(request): add last_event_id property

[granuels]

Introduce req.last_event_id to provide convenient access to the Last-Event-ID header. Closes #2580.

Summary of Changes

I added req.last_event_id to the Request class to make it easier to access the Last-Event-ID header, which is standard for Server-Sent Events. This implementation uses _header_property helper in both WSGI and ASGI files for performance and consistency. I've also included the necessary tests and a newsfragment.

Related Issues

Closes #2580

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• Applied changes to both WSGI and ASGI code paths and interfaces (where applicable).
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a33301e) to head (2b7fe66).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2587   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           64        64           
  Lines         7866      7869    +3     
  Branches      1078      1078           
=========================================
+ Hits          7866      7869    +3     

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[vytas7]

Thanks once again for the PR, it looks good for most of it.
I've requested a couple of improvements inline.

Another thing, it seems like the PR summary is heavily influenced by "AI", if not outright churned by Cursor or similar... 👿 Don't do this! It is not the end of the world, but it would be much more useful if you wrote a sentence or two yourself.

This is verbose and prone to typos (e.g., casing issues).

No, the get_header(...) name argument is not case sensitive!

[vytas7]

Could you rewrite this more as a story (what has been done), not in imperative (as docstrings)?

[vytas7]

Could we optimize this by operating on the internal headers directly?

[granuels]

I updated the PR summary and optimized the implementation to use _header_property in both WSGI and ASGI files as requested. I also updated the newsfragment to follow the narrative style. Ready for re-review!