transport: Reduce heap allocations

[arjan-bal]

Benchmarks

# Test command
$  go run benchmark/benchmain/main.go -benchtime=60s -workloads=unary \
   -compression=off -maxConcurrentCalls=200 -trace=off \
   -reqSizeBytes=100 -respSizeBytes=100 -networkMode=Local -resultFile="${RUN_NAME}"

$ go run benchmark/benchresult/main.go unary-before unary-after
               Title       Before        After Percentage
            TotalOps      7801951      7889246     1.12%
             SendOps            0            0      NaN%
             RecvOps            0            0      NaN%
            Bytes/op     10005.90      9911.48    -0.94%
           Allocs/op       146.91       143.91    -2.04%
             ReqT/op 104026013.33 105189946.67     1.12%
            RespT/op 104026013.33 105189946.67     1.12%
            50th-Lat   1.375183ms   1.360319ms    -1.08%
            90th-Lat   2.293816ms   2.249015ms    -1.95%
            99th-Lat   3.162307ms    3.13568ms    -0.84%
             Avg-Lat   1.536462ms   1.519465ms    -1.11%
           GoVersion     go1.24.8     go1.24.8
         GrpcVersion   1.77.0-dev   1.77.0-dev

RELEASE NOTES: N/A

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.98%. Comparing base (f448a97) to head (6117533).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8668   +/-   ##
=======================================
  Coverage   81.97%   81.98%           
=======================================
  Files         417      417           
  Lines       40788    40804   +16     
=======================================
+ Hits        33435    33452   +17     
+ Misses       5991     5966   -25     
- Partials     1362     1386   +24     

Files with missing lines	Coverage Δ
internal/transport/http2_client.go	92.22% <100.00%> (+0.15%)	⬆️
internal/transport/transport.go	90.72% <100.00%> (ø)
preloader.go	63.63% <ø> (+5.30%)	⬆️
rpc_util.go	82.09% <100.00%> (ø)
stream.go	81.85% <100.00%> (+0.24%)	⬆️

... and 26 files with indirect coverage changes

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