add null-safety checks in ValidateController.ProcessResource()

[v-rachitsh]

Fixes #5627

Related issues

AB#198060

Summary

Adds null-safety checks in ValidateController.ProcessResource() to prevent NullReferenceException when a Parameters resource is missing the expected "resource" parameter or has a null Parameter collection.

This is the same class of bug fixed in PR #5115 for ParameterCompatibleFilter.ParseResource(), but in a separate code path.

Root Cause

ProcessResource() at line 74 called .Find(...).Resource without null-checking the result of .Find(). If no parameter named "resource" exists, .Find() returns null and accessing .Resource throws NullReferenceException.

Additionally, parameterResource.Parameter was accessed without null-checking, which would also throw if the Parameter collection itself is null.

Changes

ValidateController.cs

• Added ?. null-conditional operator on resource.TypeName check
• Added ?. null-conditional operator on parameterResource.Parameter access
• Split the .Find(...).Resource chain into separate null-safe steps matching the pattern from PR Fix NullReferenceException in ParameterCompatibleFilter.ParseResource() during resource validation #5115

ValidateControllerTests.cs

• Added 3 new test cases:
  • GivenParametersWithNullParameterCollection_WhenValidating_ThenShouldNotThrowNullReferenceException
  • GivenParametersWithMissingResourceParameter_WhenValidating_ThenShouldNotThrowNullReferenceException
  • GivenParametersWithNullResourceParameter_WhenValidating_ThenShouldNotThrowNullReferenceException

Verification

Structural verification: the fix follows the exact same null-safety pattern established in PR #5115 for ParameterCompatibleFilter.cs. Each .Find() result is now checked for null before accessing .Resource.

Files Changed

• src/Microsoft.Health.Fhir.Shared.Api/Controllers/ValidateController.cs — Added null-conditional operators and null-safe Find result handling
• src/Microsoft.Health.Fhir.Shared.Api.UnitTests/Controllers/ValidateControllerTests.cs — Added 3 regression tests for null Parameters edge cases

Verification

• Fix follows identical pattern from PR Fix NullReferenceException in ParameterCompatibleFilter.ParseResource() during resource validation #5115 (ParameterCompatibleFilter)
• 3 new unit tests cover all null-dereference paths

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.54%. Comparing base (318be27) to head (5e0a469).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5671      +/-   ##
==========================================
+ Coverage   77.51%   77.54%   +0.03%     
==========================================
  Files        1005     1005              
  Lines       36943    36946       +3     
  Branches     5601     5603       +2     
==========================================
+ Hits        28635    28649      +14     
+ Misses       6948     6936      -12     
- Partials     1360     1361       +1     

see 14 files with indirect coverage changes

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