SPDX 2.3 conversion fails for valid primaryPackagePurpose: "OPERATING-SYSTEM"

### Description
CycloneDX CLI fails to convert a valid SPDX 2.3 JSON document when a package contains the following field:
"primaryPackagePurpose": "OPERATING-SYSTEM"

OPERATING-SYSTEM is the valid value defined by the SPDX 2.3 specification.

The CLI throws an unhandled System.Text.Json.JsonException while deserializing the SPDX document.

Replacing the valid SPDX value: `OPERATING-SYSTEM` with the non-standard value: `OPERATING_SYSTEM` allows the conversion to proceed.

### Environment
CycloneDX CLI version: `0.32.0`
Platform: `Linux x64 / WSL Ubuntu 24.04`
Input format: `SPDX JSON 2.3`
Output format: `CycloneDX JSON`
Requested output version: `CycloneDX 1.6`

### Command
```
./cyclonedx-linux-x64 convert \
--input-format spdxjson \
--output-format json \
--output-version v1_6 \
--input-file minimal.spdx.json \
--output-file output.cdx.json
```

### Minimal reproducer
```
{
"spdxVersion": "SPDX-2.3",
"dataLicense": "CC0-1.0",
"SPDXID": "SPDXRef-DOCUMENT",
"name": "operating-system-test",
"documentNamespace": "https://example.com/spdx/operating-system-test",
"creationInfo": {
"created": "2026-07-14T08:00:00Z",
"creators": [
"Tool: minimal-reproducer"
]
},
"packages": [
{
"name": "Example Linux",
"SPDXID": "SPDXRef-OperatingSystem",
"versionInfo": "1.0",
"downloadLocation": "NONE",
"filesAnalyzed": false,
"licenseConcluded": "NOASSERTION",
"licenseDeclared": "NOASSERTION",
"primaryPackagePurpose": "OPERATING-SYSTEM"
}
],
"relationships": [
{
"spdxElementId": "SPDXRef-DOCUMENT",
"relationshipType": "DESCRIBES",
"relatedSpdxElement": "SPDXRef-OperatingSystem"
}
]
}
```

### Actual result

The conversion terminates with the following exception:
```
Unhandled exception: System.Text.Json.JsonException: The JSON value could not be converted to System.Nullable`1[CycloneDX.Spdx.Models.v2_3.PrimaryPackagePurposeType]. Path: $.packages[0].primaryPackagePurpose
at System.Text.Json.ThrowHelper.ThrowJsonException(String message)
at System.Text.Json.Serialization.Converters.EnumConverter`1.Read(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options)
at System.Text.Json.Serialization.Converters.NullableConverter`1.Read(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options)
at System.Text.Json.Serialization.Metadata.JsonPropertyInfo`1.ReadJsonAndSetMember(Object obj, ReadStack& state, Utf8JsonReader& reader)
at System.Text.Json.Serialization.Converters.ObjectDefaultConverter`1.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value)
at System.Text.Json.Serialization.JsonConverter`1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value, Boolean& isPopulatedValue)
at System.Text.Json.Serialization.JsonCollectionConverter`2.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, TCollection& value)
at System.Text.Json.Serialization.JsonConverter`1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value, Boolean& isPopulatedValue)
at System.Text.Json.Serialization.Metadata.JsonPropertyInfo`1.ReadJsonAndSetMember(Object obj, ReadStack& state, Utf8JsonReader& reader)
at System.Text.Json.Serialization.Converters.ObjectDefaultConverter`1.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value)
at System.Text.Json.Serialization.JsonConverter`1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value, Boolean& isPopulatedValue)
at System.Text.Json.Serialization.JsonConverter`1.ReadCore(Utf8JsonReader& reader, T& value, JsonSerializerOptions options, ReadStack& state)
at System.Text.Json.Serialization.Metadata.JsonTypeInfo`1.ContinueDeserialize[TReadBufferState,TStream](TReadBufferState& bufferState, JsonReaderState& jsonReaderState, ReadStack& readStack, T& value)
at System.Text.Json.Serialization.Metadata.JsonTypeInfo`1.DeserializeAsync[TReadBufferState,TStream](TStream utf8Json, TReadBufferState bufferState, CancellationToken cancellationToken)
at CycloneDX.Spdx.Serialization.JsonSerializer.DeserializeAsync(Stream jsonStream)
at CycloneDX.Cli.CliUtils.InputBomHelper(String filename, ConvertFormat format)
at CycloneDX.Cli.Commands.ConvertCommand.Convert(ConvertCommandOptions options)
```

### Expected result

The CLI should successfully deserialize the SPDX 2.3 document and convert the package to a CycloneDX component.

### Analysis

The SPDX model defines the C# enum member as:

`OPERATING_SYSTEM`

while the SPDX 2.3 JSON representation requires:

`OPERATING-SYSTEM`

The SPDX JSON serializer currently registers `HyphenToUnderscoreEnumConverter` for some enum types, such as:
```
HyphenToUnderscoreEnumConverter
HyphenToUnderscoreEnumConverter
```

However, no equivalent converter appears to be registered for:

`PrimaryPackagePurposeType`

The generic `JsonStringEnumConverter` therefore attempts to match `OPERATING-SYSTEM` directly against the C# enum member `OPERATING_SYSTEM` and fails.

The issue likely originates in `CycloneDX.Spdx.Serialization.JsonSerializer` from the `cyclonedx-dotnet-library`, rather than in the CLI conversion logic itself.

### Possible fix

Register the existing converter for `PrimaryPackagePurposeType` before the generic `JsonStringEnumConverter`:

```
options.Converters.Add(
new HyphenToUnderscoreEnumConverter());
```

A regression test should verify both directions:

Deserializing:
```
"primaryPackagePurpose": "OPERATING-SYSTEM"
```

into:

```
PrimaryPackagePurposeType.OPERATING_SYSTEM
```
Serializing the enum back to the SPDX-compliant value:
`"primaryPackagePurpose": "OPERATING-SYSTEM"`
Workaround

As a temporary workaround, replacing:

`"primaryPackagePurpose": "OPERATING-SYSTEM"`

with:

`"primaryPackagePurpose": "OPERATING_SYSTEM"`

allows the conversion to complete, but the resulting temporary SPDX input is not compliant with the SPDX 2.3 specification.