Use format_ident! instead of Ident::new(&format!(...)) in model_ex (#3119)

[upuddu]

Small step toward #3119, which asks to clean up the macros one category at a time. This handles the "manually constructed Idents" category: the two Ident::new(&format!(...), span) calls in sea-orm-macros/src/derives/model_ex.rs become format_ident!(..., span = ...), matching how idents are already built elsewhere in the crate. Purely a readability change — the generated tokens are identical.

[tyt2y3]

Thanks @upuddu — but master has moved past this. The {model}Ex line is already format_ident! (converted independently), and the Reverse construction this touched has since been refactored, hence the conflict. Master deliberately uses the call-site span for the generated {model}Ex ident, so re-adding model.span() isn't a change we want. One item from #3119's category does remain — the Ident::new(&format!("{forward}Reverse"), ...) at model_ex.rs:292 — worth a fresh one-liner if you'd like to keep chipping at #3119.