Fix short reads in fixed-length deserialization

[Caideyipi]

Description

• Add FileChannel readFully helpers and use readFully/DataInputStream.readFully for fixed-length deserialization paths.
• Handle EOF explicitly for one-byte enum/category deserialization.
• Add regression tests for short reads and truncated payloads.

Verification

• .\mvnw.cmd -pl iotdb-core/node-commons -Dtest=IOUtilsTest test
• .\mvnw.cmd -pl iotdb-core/node-commons,iotdb-core/calc-commons,iotdb-core/confignode,iotdb-core/datanode spotless:apply
• git diff --check
• Static grep audit found no remaining obvious same-pattern values()[stream.read()] or fixed-length read comparison cases outside excluded areas.

Datanode focused tests were attempted, but local Windows verification was blocked by page-file/native-memory failures and existing datanode generated-source compilation issues unrelated to this patch.