fix: nullable Error dereference after ES client 9.3.3 upgrade

[reakaleek]

Summary

• Adds null-conditional operator (?.) on ElasticsearchServerError.Error in ChangesGateway and SharedPointInTimeManager
• The ES client 9.3.3 upgrade (Bump Elastic.Clients.Elasticsearch from 9.3.0 to 9.3.3 #2946) made Error nullable but missed these two files, causing 5 CS8602 compilation errors that broke the pre-release workflow

Test plan

• dotnet build src/services/Elastic.Documentation.Search/ passes with 0 errors, 0 warnings
• Pre-release workflow succeeds after merge

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between bf51ad5 and 38cabb7.

📒 Files selected for processing (2)

• src/services/Elastic.Documentation.Search/ChangesGateway.cs
• src/services/Elastic.Documentation.Search/SharedPointInTimeManager.cs

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📝 Walkthrough

Walkthrough

This pull request adds null-safe access operators (?.) to Elasticsearch error-field handling in two files. The changes ensure that accessing nested properties within response.ElasticsearchServerError does not throw null reference exceptions when intermediate properties are missing. Error-message construction now includes fallback values (?? "Unknown") for cases where error details are partially absent.

Suggested labels

fix

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: fixing nullable Error dereference issues after the ES client 9.3.3 upgrade.
Description check	✅ Passed	The description is directly related to the changeset, explaining the null-conditional operator additions and the ES client upgrade context that motivated them.

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✨ Finishing Touches

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch noble-birthday

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