Downgrade HL7 SDK v5 to v4 for STU3, R4, and R4B

[mikaelweave]

Description

Downgrade Firely Packages for STU3, R4, and R4B Due to Performance Issues

Summary

This PR downgrades the Firely (HL7.Fhir) packages for the STU3, R4, and R4B FHIR versions. The change is necessary due to significant performance regressions observed with the newer package versions—specifically, a 2.8x increase in CPU consumption during typical workloads.

Details

• Updated project files to conditionally use legacy Firely packages for STU3, R4, and R4B.
• Adjusted preprocessor directives and test logic to support both legacy and current package scenarios.
• All affected E2E and integration tests have been updated to ensure compatibility with the downgraded packages.

Rationale

Recent benchmarks revealed that the latest Firely packages introduce unacceptable CPU overhead, impacting server scalability and cost. Until these issues are resolved upstream, we are reverting to the previous, more performant versions for the affected FHIR versions.

Testing

• All unit and E2E tests pass for STU3, R4, and R4B with the downgraded packages.

Related issues

AB#154845

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)

Patch|Skip|Feature|Breaking (reason)

[mikaelweave]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[SergeyGaluzo]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).