[GRPC] Optimize source conversion in gRPC search hits using zero-copy BytesRef

[karenyrx]

Description

Current Behavior
The current implementation of gRPC search hit source conversion in SearchHitProtoUtils.processSource() always uses ByteString.copyFrom() to create Protocol Buffer byte strings, which always creates a defensive copy of the underlying byte array for every search hit, regardless of the source type. This results in unnecessary memory copying and allocation overhead.

Proposed Enhancement
This PR implements a zero-copy optimization for BytesArray sources, which:
a) Detects when the source is a BytesArray instance and using UnsafeByteOperations.unsafeWrap() for BytesArray sources
b) Maintains backward compatibility by falling back to ByteString.copyFrom() for other BytesReference implementations

Expected benefits
This should improve:

• serialization latency for large documents
• better memory efficiency - by eliminating unecessary memory allocation for BytesArray sources
• improved CPU performance - by removing memory copy operations

Related Issues

Resolves #19310

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

❌ Gradle check result for f87cce9: FAILURE

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[github-actions]

✅ Gradle check result for b13b325: SUCCESS

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.82%. Comparing base (6c4f3d8) to head (47e55dc).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...rpc/proto/response/search/SearchHitProtoUtils.java	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #19280      +/-   ##
============================================
- Coverage     72.88%   72.82%   -0.07%     
+ Complexity    69841    69798      -43     
============================================
  Files          5673     5673              
  Lines        320756   320756              
  Branches      46370    46370              
============================================
- Hits         233796   233590     -206     
- Misses        68102    68272     +170     
- Partials      18858    18894      +36     

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✅ Gradle check result for edae7f3: SUCCESS

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❌ Gradle check result for dfa762f: FAILURE

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❌ Gradle check result for 8b9e86a: FAILURE

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❌ Gradle check result for caf9cab: null

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❌ Gradle check result for eec47c3: FAILURE

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✅ Gradle check result for 47e55dc: SUCCESS