Omit null mimeType from ResourceContents JSON serialization

[Copilot]

ResourceContents.Converter.Write unconditionally wrote "mimeType": null when MimeType was unset, violating the MCP schema where the property is optional.

Changes

• ResourceContents.cs: Guard the mimeType write with a null check, consistent with how _meta is already handled in the same method
• ResourceContentsTests.cs: Add tests asserting mimeType is absent from serialized output when null, for both TextResourceContents and BlobResourceContents

// Before
writer.WriteString("mimeType", value.MimeType); // always written, even as null

// After
if (value.MimeType is not null)
{
    writer.WriteString("mimeType", value.MimeType);
}

Original prompt

In src/ModelContextProtocol.Core/Protocol/ResourceContents.cs, the custom JsonConverter for ResourceContents unconditionally writes the mimeType property in the Write method (line 161):

writer.WriteString("mimeType", value.MimeType);

However, MimeType is declared as string? (nullable/optional) on line 50:

public string? MimeType { get; set; }

This means that when MimeType is null, the serializer will still write "mimeType": null into the JSON output. Since the property is optional per the MCP schema, it should be omitted entirely when null, consistent with how Meta is handled (lines 173-177).

Fix: Wrap the mimeType write in a null check, similar to how _meta is handled:

if (value.MimeType is not null)
{
    writer.WriteString("mimeType", value.MimeType);
}

This change should be made in the Write method of the ResourceContents.Converter class at line 161 of src/ModelContextProtocol.Core/Protocol/ResourceContents.cs.

This pull request was created from Copilot chat.

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