GCC: Keep increase path monotonic

[uartnet]

The steady-state increase branch caps its result at 1.5 times the received rate. When that cap is below the current target, the increase operation can unexpectedly reduce the target even though the controller remains in the increase state. The other increase branch already guards this invariant.

Keep the target unchanged when the computed rate would be lower, and cover the regression with a focused test.

Validated with go test ./... and go test -race ./pkg/gcc.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.50%. Comparing base (9a8ca49) to head (afb564a).

Files with missing lines	Patch %	Lines
pkg/gcc/rate_controller.go	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #438      +/-   ##
==========================================
+ Coverage   80.27%   80.50%   +0.22%     
==========================================
  Files          88       88              
  Lines        4624     4627       +3     
==========================================
+ Hits         3712     3725      +13     
+ Misses        732      720      -12     
- Partials      180      182       +2     

Flag	Coverage Δ
go	80.50% <75.00%> (+0.22%)	⬆️

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