Hide empty snippets for full-file diagnostics (#19653)

Summary
--

This is the other commit I wanted to spin off from #19415, currently
stacked on #19644.

This PR suppresses blank snippets for empty ranges at the very beginning
of a file, and for empty ranges in non-existent files. Ruff includes
empty ranges for IO errors, for example.


f4e93b6335/crates/ruff_linter/src/message/text.rs (L100-L110)

The diagnostics now look like this (new snapshot test):

```
error[test-diagnostic]: main diagnostic message
--> example.py:1:1                             
```

Instead of [^*]

```
error[test-diagnostic]: main diagnostic message
--> example.py:1:1
 |
 |
```

Test Plan
--

A new `ruff_db` test showing the expected output format

[^*]: This doesn't correspond precisely to the example in the PR because
of some details of the diagnostic builder helper methods in `ruff_db`,
but you can see another example in the current version of the summary in
#19415.

crates/ruff_annotate_snippets/src/snippet.rs

@@ -124,6 +124,7 @@ pub struct Annotation<'a> {
     pub(crate) range: Range<usize>,
     pub(crate) label: Option<&'a str>,
     pub(crate) level: Level,
+    pub(crate) is_file_level: bool,
 }
 
 impl<'a> Annotation<'a> {
@@ -131,6 +132,11 @@ impl<'a> Annotation<'a> {
         self.label = Some(label);
         self
     }
+
+    pub fn is_file_level(mut self, yes: bool) -> Self {
+        self.is_file_level = yes;
+        self
+    }
 }
 
 /// Types of annotations.
@@ -165,6 +171,7 @@ impl Level {
             range: span,
             label: None,
             level: self,
+            is_file_level: false,
         }
     }
 }