Improve help output for scripts (#13445)

# Description
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Currently the parser and the documentation generation use the signature
of the command, which means that it doesn't pick up on the changed name
of the `main` block, and therefore shows the name of the command as
"main" and doesn't find the subcommands. This PR changes the
aforementioned places to use the block signature to fix these issues.
This closes #13397. Incidentally it also causes input/output types to be
shown in the help, which is kinda pointless for scripts since they don't
operate on structured data but maybe not worth the effort to remove.

# User-Facing Changes
<!-- List of all changes that impact the user experience here. This
helps us keep track of breaking changes. -->
```
# example.nu
export def main [] { help main }
export def 'main sub' [] { print 'sub' }
```
Before:

![image](https://github.com/user-attachments/assets/49fdcf8d-e56a-4c27-b7c8-7d2902c2a807)

![image](https://github.com/user-attachments/assets/4d1f4faa-5928-4269-b0b5-fd654563bb8b)


After:

![image](https://github.com/user-attachments/assets/a7232a1f-f997-4988-808c-8fa957e39bae)

![image](https://github.com/user-attachments/assets/c5628dc6-69b5-443a-b103-9e5faa9bb4ba)

# Tests
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> **Note**
> from `nushell` you can also use the `toolkit` as follows
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Tests are still missing for the subcommands and the input/output types

---------

Co-authored-by: Stefan Holderbach <sholderbach@users.noreply.github.com>

crates/nu-engine/src/documentation.rs

@@ -25,7 +25,9 @@ pub fn get_full_help(
     // execution.
     let stack = &mut stack.start_capture();
 
-    let signature = command.signature().update_from_command(command);
+    let signature = engine_state
+        .get_signature(command)
+        .update_from_command(command);
 
     get_documentation(
         &signature,

crates/nu-parser/src/parser.rs

@@ -949,7 +949,7 @@ pub fn parse_internal_call(
     let _ = working_set.add_span(call.head);
 
     let decl = working_set.get_decl(decl_id);
-    let signature = decl.signature();
+    let signature = working_set.get_signature(decl);
     let output = signature.get_output_type();
 
     // storing the var ID for later due to borrowing issues

crates/nu-protocol/src/engine/state_working_set.rs

@@ -5,7 +5,7 @@ use crate::{
         StateDelta, Variable, VirtualPath, Visibility,
     },
     BlockId, Category, CompileError, Config, DeclId, FileId, GetSpan, Module, ModuleId, ParseError,
-    ParseWarning, Span, SpanId, Type, Value, VarId, VirtualPathId,
+    ParseWarning, Signature, Span, SpanId, Type, Value, VarId, VirtualPathId,
 };
 use core::panic;
 use std::{
@@ -710,6 +710,14 @@ impl<'a> StateWorkingSet<'a> {
         }
     }
 
+    pub fn get_signature(&self, decl: &dyn Command) -> Signature {
+        if let Some(block_id) = decl.block_id() {
+            *self.get_block(block_id).signature.clone()
+        } else {
+            decl.signature()
+        }
+    }
+
     pub fn find_commands_by_predicate(
         &self,
         predicate: impl Fn(&[u8]) -> bool,

tests/shell/mod.rs

@@ -334,3 +334,69 @@ fn source_empty_file() {
         assert!(actual.out.is_empty());
     })
 }
+
+#[test]
+fn main_script_help_uses_script_name1() {
+    // Note: this test is somewhat fragile and might need to be adapted if the usage help message changes
+    Playground::setup("main_filename", |dirs, sandbox| {
+        sandbox.mkdir("main_filename");
+        sandbox.with_files(&[FileWithContent(
+            "script.nu",
+            r#"def main [] {}
+            "#,
+        )]);
+        let actual = nu!(cwd: dirs.test(), pipeline("nu script.nu --help"));
+        assert!(actual.out.contains("> script.nu"));
+        assert!(!actual.out.contains("> main"));
+    })
+}
+
+#[test]
+fn main_script_help_uses_script_name2() {
+    // Note: this test is somewhat fragile and might need to be adapted if the usage help message changes
+    Playground::setup("main_filename", |dirs, sandbox| {
+        sandbox.mkdir("main_filename");
+        sandbox.with_files(&[FileWithContent(
+            "script.nu",
+            r#"def main [foo: string] {}
+            "#,
+        )]);
+        let actual = nu!(cwd: dirs.test(), pipeline("nu script.nu"));
+        assert!(actual.err.contains("Usage: script.nu"));
+        assert!(!actual.err.contains("Usage: main"));
+    })
+}
+
+#[test]
+fn main_script_subcommand_help_uses_script_name1() {
+    // Note: this test is somewhat fragile and might need to be adapted if the usage help message changes
+    Playground::setup("main_filename", |dirs, sandbox| {
+        sandbox.mkdir("main_filename");
+        sandbox.with_files(&[FileWithContent(
+            "script.nu",
+            r#"def main [] {}
+            def 'main foo' [] {}
+            "#,
+        )]);
+        let actual = nu!(cwd: dirs.test(), pipeline("nu script.nu foo --help"));
+        assert!(actual.out.contains("> script.nu foo"));
+        assert!(!actual.out.contains("> main foo"));
+    })
+}
+
+#[test]
+fn main_script_subcommand_help_uses_script_name2() {
+    // Note: this test is somewhat fragile and might need to be adapted if the usage help message changes
+    Playground::setup("main_filename", |dirs, sandbox| {
+        sandbox.mkdir("main_filename");
+        sandbox.with_files(&[FileWithContent(
+            "script.nu",
+            r#"def main [] {}
+            def 'main foo' [bar: string] {}
+            "#,
+        )]);
+        let actual = nu!(cwd: dirs.test(), pipeline("nu script.nu foo"));
+        assert!(actual.err.contains("Usage: script.nu foo"));
+        assert!(!actual.err.contains("Usage: main foo"));
+    })
+}