build: remove Guava dependency

[andygrove]

Which issue does this PR close?

N/A. This is dependency cleanup and does not fix a user-visible bug or add an enhancement.

Rationale for this change

Comet only used a small set of Guava utilities that have direct JDK equivalents. Removing those usages avoids carrying, shading, and relocating Guava and its failureaccess companion artifact in Comet's build and packaged JAR.

What changes are included in this PR?

• Replace Guava utilities with JDK equivalents for object hashing, stream copying and closing, unsigned-long test values, and classpath scanning.
• Remove the direct Guava and failureaccess dependencies, Guava shade configuration, relocation rules, and duplicate-class exception.
• Update contributor documentation examples to use java.util.Objects.

Spark's explicit Guava exclusion remains unchanged because it controls Spark's transitive dependency rather than Comet's direct dependency.

How are these changes tested?

• make core
• ./mvnw package -DskipTests (Spark 4.1 / Scala 2.13)
• ./mvnw -Pspark-3.5 package -DskipTests (Spark 3.5 / Scala 2.12)
• Focused plan-equality and shuffle suites: 6 passed, 1 Spark-4.2-only test canceled
• Parquet V1 suite with Spark 3.5: 79 passed, 2 canceled, 1 ignored
• Verified the Maven dependency tree contains no guava or failureaccess artifacts
• Verified the shaded JAR contains no Guava or relocated-Guava classes