fix(plugin-mcp): omit body on GET/HEAD and fix schema fallback partial()

[SakshixSingh]

fixes #17125

What?

Fixes two MCP plugin bugs:

1. GET/HEAD requests no longer pass a body when constructing the Request object
2. Schema conversion fallback returns z.object({}) instead of z.record(z.any()) so .partial() works in update tools

Why?

• MCP uses GET for the SSE stream; passing a body causes Request with GET/HEAD method cannot have body (500)
• When jsonSchemaToZod fails on complex schemas, the fallback z.record() crashes update tools that call .partial()

How?

• createRequest.ts: conditionally omit body/duplex for GET and HEAD
• convertCollectionSchemaToZod.ts: use z.object({}) as permissive fallback
• Added unit tests for both fixes