Use anyOf for the mixed item type in link_work_items

What does this MR do and why?

The link_work_items tool's inputSchema declared work_items_ids.items with an array-form type, ["integer", "string"]. That is valid JSON Schema, but several MCP clients read type as a single string and either reject the tool or drop the constraint; MCP Inspector flags it.

This MR expresses the same constraint as anyOf: [{type: integer}, {type: string}], which every known client accepts. It sits inside items, so the root schema's additionalProperties handling (and the list_tools request spec that checks it) is unaffected.

I audited the other tools as the issue asked: no other hand-written tool under app/services/mcp/tools uses the array form, and Base::ApiTool#parse_type always produces a single type string for API-derived schemas.

References

• Resolves #628820 (closed)

Differences

Before: tools/list returns "items": {"type": ["integer", "string"]} for work_items_ids.

After: It returns "items": {"anyOf": [{"type": "integer"}, {"type": "string"}]}.

How to set up and validate locally

1. Call tools/list on the MCP endpoint and find link_work_items.
2. inputSchema.properties.work_items_ids.items carries anyOf; loading the tool in MCP Inspector no longer warns.

MR acceptance checklist

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