fix(docker): use protoc v25 for ml_training_service CUDA build

Ubuntu 22.04's protobuf-compiler (v3.12) lacks proto3 optional field
support. Install protoc v25.1 from GitHub releases instead, which
matches the protoc version available in Debian bookworm-based images
used by the other 5 services.

Validated: docker build completes successfully, binary starts with
ml_training_service --help.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

services/ml_training_service/Dockerfile

@@ -18,12 +18,19 @@ RUN apt-get update && apt-get install -y \
 ENV PATH="/root/.cargo/bin:${PATH}"
 
 # Install build dependencies including CUDA toolkit
+# Note: protobuf-compiler from Ubuntu 22.04 is v3.12 which lacks proto3 optional support.
+# Install protoc v25 from GitHub releases instead.
 RUN apt-get update && apt-get install -y \
     pkg-config \
     libssl-dev \
-    protobuf-compiler \
     nvidia-cuda-toolkit \
-    && rm -rf /var/lib/apt/lists/*
+    unzip \
+    && rm -rf /var/lib/apt/lists/* \
+    && curl -sSL https://github.com/protocolbuffers/protobuf/releases/download/v25.1/protoc-25.1-linux-x86_64.zip \
+       -o /tmp/protoc.zip \
+    && unzip -q /tmp/protoc.zip -d /usr/local bin/protoc 'include/*' \
+    && rm /tmp/protoc.zip \
+    && chmod +x /usr/local/bin/protoc
 
 # Set working directory
 WORKDIR /build