perf: cache offsetBufferAddress in CometPlainVector for variable-width vectors

[0lai0]

Which issue does this PR close?

Closes #4280

Rationale for this change

CometPlainVector already caches the value buffer address, but variable-width reads still fetched the offset buffer address on every getUTF8String and getBinary call. Caching the offset buffer address avoids repeated Arrow buffer lookups on hot per-row paths.

What changes are included in this PR?

• Cache offsetBufferAddress in CometPlainVector for variable-width vectors
• Use the cached offset buffer address in getUTF8String and getBinary
• Add targeted JUnit coverage for variable-width string and binary reads

How are these changes tested?

./mvnw test-compile surefire:test \
  -pl spark \
  -Dtest=org.apache.comet.vector.TestCometPlainVector,org.apache.comet.parquet.TestColumnReader \
  -DfailIfNoTests=false \
  -Dscalastyle.skip=true