Clean error prone Warnings (Fixes #501 partially)

[ekagra1602]

Summary

Describe the goal of this PR. Mention any related Issue numbers.

This PR fixes issue #501 partially.

Fix Details

This PR addresses 30 low-risk Error Prone warnings throughout the codebase to improve code quality and prevent potential issues.

Changes Summary:

DefaultCharset Fixes (8)

• Added explicit UTF-8 charset specification instead of using platform defaults in:
  • MetadataSerializer.java (toJsonStr, fromJsonStr)
  • BulkApiRequestParser.java (ByteString.copyFromUtf8)
  • SpanFormatter.java (ByteString conversion)
  • ChunkSchema.java (String constructor)
  • RedactionStoredFieldVisitor.java (getBytes)
  • JsonUtil.java (getBytes)

FutureReturnValueIgnored Fixes (7)

• Added explicit handling for ignored Future return values by storing in var unused variables:
  • BulkIngestApi.java (thenRun)
  • CachingChunkManager.java (submit)
  • ReadOnlyChunkImpl.java (updateAssignmentState)
  • DiskOrMessageCountBasedRolloverStrategy.java (scheduleAtFixedRate)
  • LuceneIndexStoreImpl.java (scheduleWithFixedDelay - both instances)

EqualsGetClass Improvements (7)

• Replaced getClass() comparison with more flexible instanceof pattern:
  • AstraMetadata.java
  • ChunkInfo.java
  • SearchResult.java
  • LuceneFieldDef.java
  • SearchMetadata.java
  • ChunkSchema.java
  • RecoveryNodeMetadata.java

Type Conversion Improvements (5)

• Added explicit casts for numeric type conversions:
  • FieldType.java (int literals to 0L, 0.0f)
  • LuceneIndexStoreImpl.java (ramBufferSizeMb to double)
  • AstraMergeScheduler.java (long to double)
  • RecoveryService.java (two long calculations to double)
  • AstraKafkaConsumer.java (int multiplication to long - 3000L)

Other Fixes (3)

• Fixed BadImport issue in LogIndexSearcherImpl.java (SortField.Type qualification)
• Fixed PreconditionsExpensiveString in AstraKafkaConsumer.java (removed String.format)
• Fixed NotJavadoc in ZipkinService.java (/** to /*)

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

cc @baroquebobcat for review, please let me know if you'd like anything adjusted.

[mansu]

Let's make sure this change is backwards compatible. Ensure no serde warnings when we deploy.