[Bug - MCP server] Tool output schemas are not generated for primitives or "simple" types

Bug description

This tool will silently not get an output schema

@McpTool(generateOutputSchema = true)
public int returnsInt() {
     return 1;
}

Environment

Spring AI version: 2.0.0

Expected behavior

Generate this output schema

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "integer",
  "format": "int32"
}

Details

Primitive types are filtered here

https://github.com/spring-projects/spring-ai/blob/573d6d342e047d69f16fb3959568e9d1766e24e8/mcp/mcp-annotations/src/main/java/org/springframework/ai/mcp/annotation/provider/tool/SyncMcpToolProvider.java#L124-L131

I know the schema is simple, but what's the reason to not generate it? It's useful for the LLM to know what type a tool returns, even when the type is simple.

I tried to git blame this to see if I could determine the reason this filtering was added, but it was bulk imported as part of a 50k LOC commit.

Probably if generateOutputSchema is set to true, and the code decides that it will not be honoured, then there should be a warning logged

Start in mcp/mcp-annotations/src/main/java/org/springframework/ai/mcp/annotation/provider/tool/SyncMcpToolProvider.java around lines 124-131, where primitive types are filtered. Check how generateOutputSchema is handled for int and other simple return types, then verify that returnsInt() produces the expected integer JSON schema when the flag is true.