Restrict LogRecord creation access outside opentelemetry_sdk crate.

[lalitb]

Changes

The only way to create LogRecord outside the opentelemetry_sdk should be using Logger::create_log_record() method.
So,

• Removed the Default implementation for LogRecord.
• Added a crate-level constructor LogRecord::new.
• Ensured LogRecord creation is restricted to Logger::create_log_record().
• Updated tests to use LogRecord::new() or Logger::create_log_record() for record creation where applicable.

Note: It is not possible to make Default implementation private, so it was removed, and crate private constructor was added instead.

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 86.27451% with 7 lines in your changes missing coverage. Please review.

Project coverage is 77.8%. Comparing base (6a8db83) to head (fbd12e5).

Files with missing lines	Patch %	Lines
opentelemetry-proto/src/transform/logs.rs	69.2%	4 Missing ⚠️
opentelemetry-sdk/src/logs/log_processor.rs	81.8%	2 Missing ⚠️
...y-sdk/src/logs/log_processor_with_async_runtime.rs	50.0%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2549   +/-   ##
=====================================
  Coverage   77.8%   77.8%           
=====================================
  Files        122     122           
  Lines      23030   23055   +25     
=====================================
+ Hits       17931   17952   +21     
- Misses      5099    5103    +4     

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