Improve test coverage for Shared API FhirController

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Description

Added unit tests for FhirController methods to increase Shared API test coverage from 64.40% to target of 80%+.

New test coverage:

• Patch operations (JSON and FHIR Patch, conditional and non-conditional)
• Search operations (system-level, resource type, and compartment searches)
• Metadata and configuration endpoints (capabilities, SMART config, versions)

Fixed:

• Corrected GivenHttpStatusCode_WhenProcessingCustomError_ThenCorrectFhirResultShouldBeCreated test signature from async Task to void (synchronous test, no await calls)

Related issues

Addresses [issue #174924].

User Story 174924: Improve Test Coverage for Shared Api (64.40% → 80%)

Testing

Tests validated locally using xUnit test runner.

FHIR Team Checklist

• Update the title of the PR to be succinct and less than 65 characters
• Add a milestone to the PR for the sprint that it is merged (i.e. add S47)
• Tag the PR with the type of update: Bug, Build, Dependencies, Enhancement, New-Feature or Documentation
• Tag the PR with Open source, Azure API for FHIR (CosmosDB or common code) or Azure Healthcare APIs (SQL or common code) to specify where this change is intended to be released.
• Tag the PR with Schema Version backward compatible or Schema Version backward incompatible or Schema Version unchanged if this adds or updates Sql script which is/is not backward compatible with the code.
• When changing or adding behavior, if your code modifies the system design or changes design assumptions, please create and include an ADR.
• CI is green before merge
• Review squash-merge requirements

Semver Change (docs)

Skip (test-only changes)

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