Migrate to bzlmod for dependency management

Replace WORKSPACE-based dependency management with bzlmod (MODULE.bazel),
following Bazel's modern dependency management approach. This migration
aligns with Bazel 8+ best practices and prepares for the eventual
deprecation of WORKSPACE in Bazel 9.

Changes:
- Create MODULE.bazel with all external dependencies
- Migrate rules_license, bazel_skylib, rules_java, bazel_features
- Migrate rules_jvm_external and Maven dependencies to bzlmod extensions
- Update .bazelrc to enable bzlmod explicitly
- Reduce WORKSPACE.bazel to minimal stub with reference to MODULE.bazel
- Add MODULE.bazel.lock for reproducible builds
- Suppress maven repository multi-module contribution warning

All dependencies maintained at same versions:
- rules_license 1.0.0
- bazel_skylib 1.8.2
- rules_java 8.16.1
- bazel_features 1.38.0
- rules_jvm_external 6.9
- Maven: guava 33.3.1-jre, jsr305 3.0.2

---
Prompt:
```
Good work creating this demo package. But since we're starting fresh, I'd like it to be using latest Bazel style: https://bazel.build/external/migration. Implement the migration to bzlmod
```

Author: dhalperi

.bazelrc

@@ -1,2 +1,3 @@
-# Enable WORKSPACE for Bazel 8
-common --enable_workspace
+# Use bzlmod (default in Bazel 8+)
+# Note: bzlmod is enabled by default, this is just explicit
+common --enable_bzlmod

MODULE.bazel

@@ -0,0 +1,27 @@
+module(
+    name = "java_code_runner",
+    version = "0.1.0",
+)
+
+# Core Bazel dependencies
+bazel_dep(name = "rules_license", version = "1.0.0")
+bazel_dep(name = "bazel_skylib", version = "1.8.2")
+bazel_dep(name = "rules_java", version = "8.16.1")
+bazel_dep(name = "bazel_features", version = "1.38.0", repo_name = "bazel_features")
+
+# Maven dependencies via rules_jvm_external
+bazel_dep(name = "rules_jvm_external", version = "6.9")
+
+maven = use_extension("@rules_jvm_external//:extensions.bzl", "maven")
+maven.install(
+    artifacts = [
+        "com.google.guava:guava:33.3.1-jre",
+        "com.google.code.findbugs:jsr305:3.0.2",
+    ],
+    repositories = [
+        "https://repo1.maven.org/maven2",
+    ],
+    # Suppress warning about contributions from multiple modules
+    known_contributing_modules = ["java_code_runner", "protobuf"],
+)
+use_repo(maven, "maven")