mem: Remove Reader interface and export the concrete struct

[arjan-bal]

This PR changes the exported slice reader from an interface to a concrete struct.

This approach follows the precedent set by standard library packages, such as bufio's bufio.Reader. This interface was not intended for users to implement, and gRPC does not plan to provide alternative implementations. Users who require an interface for abstraction or testing can define one in their own packages.

This change provides two main advantages:

• Performance: It avoids a couple of heap allocations per stream that were previously required to hold the interface value.
• Maintainability: Adding new methods to the concrete struct is a backward-compatible change, whereas adding methods to an interface is a breaking change.

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      7883976     1.05%
             SendOps            0            0      NaN%
             RecvOps            0            0      NaN%
            Bytes/op     10005.90      9951.01    -0.54%
           Allocs/op       146.91       144.90    -1.36%
             ReqT/op 104026013.33 105119680.00     1.05%
            RespT/op 104026013.33 105119680.00     1.05%
            50th-Lat   1.375183ms   1.359194ms    -1.16%
            90th-Lat   2.293816ms   2.258941ms    -1.52%
            99th-Lat   3.162307ms   3.157381ms    -0.16%
             Avg-Lat   1.536462ms   1.520149ms    -1.06%
           GoVersion     go1.24.8     go1.24.8
         GrpcVersion   1.77.0-dev   1.77.0-dev

RELEASE NOTES:

• mem: Replace the Reader interface with a struct.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.02%. Comparing base (f448a97) to head (7d18a35).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8669      +/-   ##
==========================================
+ Coverage   81.97%   83.02%   +1.04%     
==========================================
  Files         417      417              
  Lines       40788    32294    -8494     
==========================================
- Hits        33435    26811    -6624     
+ Misses       5991     4099    -1892     
- Partials     1362     1384      +22     

Files with missing lines	Coverage Δ
internal/transport/controlbuf.go	90.04% <100.00%> (+0.15%)	⬆️
mem/buffer_slice.go	97.29% <100.00%> (+0.86%)	⬆️

... and 362 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.