perf: avoid redundant scans in BalancedPool dispatcher selection

[trivikr]

This relates to...

Performance cleanup in BalancedPool dispatcher selection.

Rationale

BalancedPool[kGetDispatcher] previously scanned the client list multiple times per dispatch: once to find an available dispatcher, again to check whether all clients were busy, then again to find a non-draining fallback index before entering the weighted selection loop.

The busy check was redundant after an available dispatcher had already been found, and the fallback selection can be tracked during the weighted loop.

Changes

Collapse dispatcher availability checks and fallback max-weight tracking into the existing weighted round-robin loop.

This keeps per-dispatch complexity at O(n) but removes up to three extra client-array scans. The improvement should be measurable with a targeted kGetDispatcher() microbenchmark, especially as upstream count grows; it may be hard to see in network-heavy benchmarks.

Features

N/A

Bug Fixes

N/A

Breaking Changes and Deprecations

N/A

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

Assisted-by: openai:gpt-5.5

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.21%. Comparing base (96fd5e9) to head (dca2afc).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5146   +/-   ##
=======================================
  Coverage   93.21%   93.21%           
=======================================
  Files         110      110           
  Lines       36247    36245    -2     
=======================================
  Hits        33787    33787           
+ Misses       2460     2458    -2     

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[mcollina]

lgtm