fix: use the default partition for the limit quota when the request partition name is empty

[SimFG]

• issue: [Bug]: Partition-level rate limiting is not working #37685

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[bigsheeper]

/lgtm

[mergify]

@SimFG E2e jenkins job failed, comment /run-cpu-e2e can trigger the job again.

[codecov]

Codecov Report

Attention: Patch coverage is 57.89474% with 8 lines in your changes missing coverage. Please review.

Project coverage is 81.04%. Comparing base (27c22d1) to head (b1b97c4).

Files with missing lines	Patch %	Lines
internal/proxy/task_insert.go	33.33%	3 Missing and 1 partial ⚠️
internal/proxy/task_upsert.go	33.33%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #38005       +/-   ##
===========================================
+ Coverage   68.87%   81.04%   +12.17%     
===========================================
  Files         289     1360     +1071     
  Lines       25481   190787   +165306     
===========================================
+ Hits        17551   154632   +137081     
- Misses       7930    30657    +22727     
- Partials        0     5498     +5498     

Components	Coverage Δ
Client	72.33% <ø> (∅)
Core	68.87% <ø> (ø)
Go	83.21% <57.89%> (∅)

Files with missing lines	Coverage Δ
internal/proxy/meta_cache.go	90.47% <100.00%> (ø)
internal/proxy/rate_limit_interceptor.go	100.00% <ø> (ø)
internal/proxy/task_insert.go	73.70% <33.33%> (ø)
internal/proxy/task_upsert.go	70.69% <33.33%> (ø)

... and 1067 files with indirect coverage changes

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• Flaky Tests Detection - Detect and resolve failed and flaky tests

[SimFG]

/run-cpu-e2e