[Geneva Exporter] Add CentralBlob decoder test utility and enhanced OTLP encoder tests

[lalitb]

Summary

Adds a test-only CentralBlob decoder utility and comprehensive unit tests for the OTLP encoder to validate encoding correctness.

While the actual correctness of encoding is validated using the validation with the binary data blob encoded using C++ encoder in mod.rs, this extends the functionality further to test the values for individual OTLP log fields, attribute types, trace context, schema generation, batching behavior, and end-to-end encoding/decoding workflows.

Changes

• New: central_blob_decoder.rs - Test-only utility for decoding CentralBlob payloads
• Enhanced: otlp_encoder.rs - Added 7 basic tests covering encoder functionality.

New unit tests:

• test_schema_caching_behavior - Validates schema reuse and cache efficiency
• test_event_name_and_batching - Tests event grouping and empty name handling
• test_comprehensive_field_encoding - Verifies all field types are properly encoded
• test_field_ordering_consistency - Ensures field order doesn't affect schema ID
• test_multiple_schemas_per_batch - Tests multiple schemas in single batch
• test_minimal_vs_maximal_logs - Covers edge cases for minimal/maximal logs
• test_timestamp_and_id_encoding - Validates timestamp and ID hex encoding

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 92.42054% with 31 lines in your changes missing coverage. Please review.

Project coverage is 49.7%. Comparing base (492701b) to head (a552ffc).

Files with missing lines	Patch %	Lines
...eneva-uploader/src/payload_encoder/otlp_encoder.rs	91.7%	21 Missing ⚠️
...loader/src/payload_encoder/central_blob_decoder.rs	93.5%	10 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #374     +/-   ##
=======================================
+ Coverage   48.4%   49.7%   +1.3%     
=======================================
  Files         69      70      +1     
  Lines       9456    9722    +266     
=======================================
+ Hits        4581    4841    +260     
- Misses      4875    4881      +6     

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