[Enhancement] Reuse MD5 digest and avoid UTF-8 length allocations

[zjncs]

Fixes #10976

Summary

• reuse one MD5 MessageDigest per thread, resetting it for each calculation
• compute UTF-8 encoded lengths without allocating temporary byte arrays on the gRPC send path
• preserve Java UTF-8 replacement behavior for unpaired surrogates

Verification

• git diff --check passed
• Maven is unavailable locally; CI should run common and proxy test suites

AI-assisted contribution; reviewed locally.