JBP_CONFIG_OPEN_JDK_JRE with only memory_calculator settings fails to stage

[stokpop]

Setting JBP_CONFIG_OPEN_JDK_JRE to tune memory calculator without specifying a JRE version causes a hard staging failure:

Failed to install JRE: failed to determine OpenJDK version from manifest:
could not parse version from JBP_CONFIG_OPEN_JDK_JRE='{ memory_calculator: { class_count: 30000 } }'

Expected behaviour (matches Ruby buildpack)

In the Ruby buildpack, JBP_CONFIG_OPEN_JDK_JRE version and memory_calculator were independent config sections. Setting only memory_calculator: was valid — the JRE version fell back to the default from config/open_jdk_jre.yml.

Fix

When JBP_CONFIG_OPEN_JDK_JRE contains no version: field, fall back to the manifest default version instead of returning an error.