Fix Icon.Theme documentation to match MCP specification

[Copilot]

The Icon.Theme property documentation incorrectly stated that custom theme identifiers are allowed. Per the MCP specification schema, theme is an enum with only two valid values: "light" and "dark".

Changes

• Updated XML doc comment on Icon.Theme to remove "custom theme identifier" claim
• Aligned description with official schema: explains "light" is for light backgrounds, "dark" for dark backgrounds
• Added note that clients should assume any-theme compatibility when not specified

- /// Can be "light", "dark", or a custom theme identifier.
- /// The value is used to specify which UI theme the icon is designed for.
+ /// Can be "light" or "dark". "light" indicates the icon is designed to be used with a light
+ /// background, and "dark" indicates the icon is designed to be used with a dark background.
+ /// If not provided, clients should assume the icon can be used with any theme.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Icon themes do not support a "custom theme identifier"</issue_title>
<issue_description>It must be light or dark. I don't know where Copilot got this idea from.

- Can be "light", "dark", or a custom theme identifier.
+ Can be "light" or "dark".
```</issue_description>

<agent_instructions>Please review the specification at https://github.com/modelcontextprotocol/modelcontextprotocol/tree/main/schema/2025-11-25 and https://github.com/modelcontextprotocol/modelcontextprotocol/tree/main/docs/specification/2025-11-25 as it applies to icons, and ensure both the shape of the Icon type and related types, as well as the comments on those types are consistent with the specification. The commenter on this issue appears to be correct that only "light" and "dark" are valid, not arbitrary custom values (at least not currently).</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Icon themes do not support a "custom theme identifier" #1089

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