fix: match rate limits by data category not envelope item type

[jamescrosswell]

Summary

Rate limits in the X-Sentry-Rate-Limits header are keyed by data category (error, monitor, log_item, metric_bucket, …) in Relay/Rust.

However since 2020 our SDK matched them against the envelope item type (event, check_in, statsd, log…). Those two only line up by coincidence for some types, so limits silently didn't match for the rest — the SDK ignored the backoff and kept sending.

• error was broken from day one: Relay has always sent error, we compared it to the item type event. (Not a Relay change — we were wrong from our first rate-limit commit.)
• monitor (check-ins) and log_item (logs) broke the moment we added those features: the category already existed in Relay, we just never mapped the item type to it.
• metric_bucket was patched in 2024 with a one-off special case rather than a real fix.

Additionally, since we'd keyed client reports the default category, if Default ever got rate limited, client reports would erroneously be dropped.

Ultimately we had two parallel notions of an item's category — a correct EnvelopeItem.DataCategory map (used for client-report accounting) and a separate raw-item-type string comparison in the rate-limiter. They were never reconciled, so every new item type whose type-string ≠ category-name reintroduced the bug.

Fix

The fix deletes the item-type comparison and matches on item.DataCategory. Any new item type now only needs its DataCategory mapping (which it needs anyway for client reports), and rate limiting follows automatically, so this class of bug can't silently reappear.

Changelog Entry

The SDK was incorrectly ignoring server rate limits for errors, check-ins, and logs

Issues

Closes #4535

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.62%. Comparing base (a9a990b) to head (1e4a12e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5412      +/-   ##
==========================================
+ Coverage   74.59%   74.62%   +0.03%     
==========================================
  Files         512      512              
  Lines       18666    18672       +6     
  Branches     3659     3657       -2     
==========================================
+ Hits        13923    13934      +11     
+ Misses       3868     3865       -3     
+ Partials      875      873       -2     

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