fix(milestone): persist due_on on create and edit (#189)

Fixes https://gitea.com/gitea/gitea-mcp/issues/187

The `due_on` argument was declared in the `milestone_write` schema but never read by `createMilestoneFn` or `editMilestoneFn`, so `opt.Deadline` was always nil and the field was silently dropped. This reuses the existing `params.GetOptionalTime` helper that already handles the analogous `deadline` field on issues and pull requests.

---
This PR was written with the help of Claude Opus 4.7

Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/189
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

operation/milestone/milestone.go

@@ -177,6 +177,7 @@ func createMilestoneFn(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallT
 	if ok {
 		opt.Description = description
 	}
+	opt.Deadline = params.GetOptionalTime(req.GetArguments(), "due_on")
 
 	client, err := gitea.ClientFromContext(ctx)
 	if err != nil {
@@ -219,6 +220,7 @@ func editMilestoneFn(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallToo
 	if ok {
 		opt.State = new(gitea_sdk.StateType(state))
 	}
+	opt.Deadline = params.GetOptionalTime(req.GetArguments(), "due_on")
 
 	client, err := gitea.ClientFromContext(ctx)
 	if err != nil {