Investigate and remove suppressed build diagnostics

[brendankowitz]

User story
As a FHIR Server maintainer, I want the diagnostics suppressed during the .NET 10 and Healthcare Shared migration to be investigated and addressed so that real compatibility, correctness, performance, and dependency issues are not hidden from CI.

The following suppressions added or retained by the migration branch require review:

• CA1873
• CA2024
• CA2025
• ASPDEPR005
• ASPDEPR006
• ASPDEPR008
• NU1510

They currently appear in CustomAnalysisRules.ruleset and/or Directory.Build.props.

Acceptance criteria

1. When each diagnostic is enabled independently, then all resulting findings are inventoried and classified as actionable, false positive, generated-code issue, or intentional compatibility exception.
2. When a finding is actionable, then the underlying code or dependency declaration is corrected with appropriate tests rather than globally suppressed.
3. When a suppression must remain, then it is narrowed to the smallest practical scope and documented with a concrete rationale and follow-up condition.
4. When remediation is complete, then redundant entries are removed from both CustomAnalysisRules.ruleset and Directory.Build.props.
5. When the repository is built for net8.0 and net10.0 with warnings treated as errors, then both builds pass without globally ignoring these diagnostics.