fix(mcp): prevent SIGSEGV in AddTool when schema is nil

[wucm667]

Bug

mcp.AddTool crashes the server with SIGSEGV when input/output struct tags trigger a nil schema.

Root Cause

In mcp/server.go, the function setSchema calls internalSchema.Resolve(...) without checking if internalSchema is nil. If the schema inference (e.g., via jsonschema.ForType) or a provided schema results in a nil value, this dereference causes a panic.

Fix

Added nil checks for internalSchema before calling Resolve in two locations within setSchema:

1. After schema generation via jsonschema.ForType.
2. After processing a provided schema (which might be nil or fail to unmarshal).

If internalSchema is nil, the function now returns a descriptive error instead of crashing.

Verification

• go test ./mcp/... passes.
• The fix prevents the crash by handling the nil case gracefully.

Closes #916.