fix: fix flaky BulkWriter rate limiter test

[thebrianchen]

The test flakes when the CI slows down, which allows enough time to pass such that the test's writes can be made without exceeding the rate limit. To fix this, I created a mock rate limiter that always rejects any request, which makes this test timing-agnostic.

[codecov]

Codecov Report

Merging #1307 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1307   +/-   ##
=======================================
  Coverage   98.51%   98.51%           
=======================================
  Files          30       30           
  Lines       18754    18760    +6     
  Branches     1450     1451    +1     
=======================================
+ Hits        18475    18481    +6     
  Misses        276      276           
  Partials        3        3           

Impacted Files	Coverage Δ
dev/src/bulk-writer.ts	99.76% <100.00%> (+<0.01%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 57dcf1c...5377d40. Read the comment docs.

[thebrianchen]

Update: also added an additional check in the BulkWriter constructor to lower the max batch size to match the starting rate if the starting rate is lower than the max batch size.