refactor(completion, parser): move custom_completion info from Expression to Signature (#15613)

Restricts custom completion from universal to internal arguments only.

Pros:
1. Less memory
2. More flexible for later customizations, e.g. #14923 

Cons:
1. limited customization capabilities, but at least covers all currently
existing features in nushell.

# Description

Mostly vibe coded by [Zed AI](https://zed.dev/ai) with a single prompt.
LGTM, but I'm not so sure @ysthakur 

# User-Facing Changes

Hopefully none.

# Tests + Formatting

+3

# After Submitting

---------

Co-authored-by: Yash Thakur <45539777+ysthakur@users.noreply.github.com>

crates/nu-engine/src/documentation.rs

@@ -648,14 +648,6 @@ impl HelpStyle {
     }
 }
 
-/// Make syntax shape presentable by stripping custom completer info
-fn document_shape(shape: &SyntaxShape) -> &SyntaxShape {
-    match shape {
-        SyntaxShape::CompleterWrapper(inner_shape, _) => inner_shape,
-        _ => shape,
-    }
-}
-
 #[derive(PartialEq)]
 enum PositionalKind {
     Required,
@@ -686,15 +678,14 @@ fn write_positional(
                 long_desc,
                 "{help_subcolor_one}\"{}\" + {RESET}<{help_subcolor_two}{}{RESET}>",
                 String::from_utf8_lossy(kw),
-                document_shape(shape),
+                shape,
             );
         }
         _ => {
             let _ = write!(
                 long_desc,
                 "{help_subcolor_one}{}{RESET} <{help_subcolor_two}{}{RESET}>",
-                positional.name,
-                document_shape(&positional.shape),
+                positional.name, &positional.shape,
             );
         }
     };
@@ -767,11 +758,7 @@ where
         }
         // Type/Syntax shape info
         if let Some(arg) = &flag.arg {
-            let _ = write!(
-                long_desc,
-                " <{help_subcolor_two}{}{RESET}>",
-                document_shape(arg)
-            );
+            let _ = write!(long_desc, " <{help_subcolor_two}{arg}{RESET}>");
         }
         if !flag.desc.is_empty() {
             let _ = write!(

crates/nu-engine/src/scope.rs

@@ -1,5 +1,5 @@
 use nu_protocol::{
-    DeclId, ModuleId, Signature, Span, SyntaxShape, Type, Value, VarId,
+    DeclId, ModuleId, Signature, Span, Type, Value, VarId,
     ast::Expr,
     engine::{Command, EngineState, Stack, Visibility},
     record,
@@ -214,7 +214,8 @@ impl<'e, 's> ScopeData<'e, 's> {
 
         // required_positional
         for req in &signature.required_positional {
-            let custom = extract_custom_completion_from_arg(self.engine_state, &req.shape);
+            let custom =
+                extract_custom_completion_from_arg(self.engine_state, &req.custom_completion);
 
             sig_records.push(Value::record(
                 record! {
@@ -233,7 +234,8 @@ impl<'e, 's> ScopeData<'e, 's> {
 
         // optional_positional
         for opt in &signature.optional_positional {
-            let custom = extract_custom_completion_from_arg(self.engine_state, &opt.shape);
+            let custom =
+                extract_custom_completion_from_arg(self.engine_state, &opt.custom_completion);
             let default = if let Some(val) = &opt.default_value {
                 val.clone()
             } else {
@@ -258,7 +260,8 @@ impl<'e, 's> ScopeData<'e, 's> {
         // rest_positional
         if let Some(rest) = &signature.rest_positional {
             let name = if rest.name == "rest" { "" } else { &rest.name };
-            let custom = extract_custom_completion_from_arg(self.engine_state, &rest.shape);
+            let custom =
+                extract_custom_completion_from_arg(self.engine_state, &rest.custom_completion);
 
             sig_records.push(Value::record(
                 record! {
@@ -285,11 +288,10 @@ impl<'e, 's> ScopeData<'e, 's> {
                 continue;
             }
 
-            let mut custom_completion_command_name: String = "".to_string();
+            let custom_completion_command_name: String =
+                extract_custom_completion_from_arg(self.engine_state, &named.custom_completion);
             let shape = if let Some(arg) = &named.arg {
                 flag_type = Value::string("named", span);
-                custom_completion_command_name =
-                    extract_custom_completion_from_arg(self.engine_state, arg);
                 Value::string(arg.to_string(), span)
             } else {
                 flag_type = Value::string("switch", span);
@@ -544,14 +546,16 @@ impl<'e, 's> ScopeData<'e, 's> {
     }
 }
 
-fn extract_custom_completion_from_arg(engine_state: &EngineState, shape: &SyntaxShape) -> String {
-    match shape {
-        SyntaxShape::CompleterWrapper(_, custom_completion_decl_id) => {
-            let custom_completion_command = engine_state.get_decl(*custom_completion_decl_id);
-            let custom_completion_command_name: &str = custom_completion_command.name();
-            custom_completion_command_name.to_string()
-        }
-        _ => "".to_string(),
+fn extract_custom_completion_from_arg(
+    engine_state: &EngineState,
+    decl_id: &Option<DeclId>,
+) -> String {
+    if let Some(decl_id) = decl_id {
+        let custom_completion_command = engine_state.get_decl(*decl_id);
+        let custom_completion_command_name: &str = custom_completion_command.name();
+        custom_completion_command_name.to_string()
+    } else {
+        "".to_string()
     }
 }