[ty] Add GitLab output format (#20155)

## Summary

This wires up the GitLab output format moved into `ruff_db` in
https://github.com/astral-sh/ruff/pull/20117 to the ty CLI.

While I was here, I made one unrelated change to the CLI docs. Clap was
rendering the escapes around the `\[default\]` brackets for the `full`
output, so I just switched those to parentheses:

```
--output-format <OUTPUT_FORMAT>
    The format to use for printing diagnostic messages

    Possible values:
    - full:    Print diagnostics verbosely, with context and helpful hints \[default\]
    - concise: Print diagnostics concisely, one per line
    - gitlab:  Print diagnostics in the JSON format expected by GitLab Code Quality reports
```

## Test Plan

New CLI test, and a manual test with `--config 'terminal.output-format =
"gitlab"'` to make sure this works as a configuration option too. I also
tried piping the output through jq to make sure it's at least valid JSON

crates/ty/src/args.rs

@@ -306,7 +306,7 @@ impl clap::Args for RulesArg {
 /// The diagnostic output format.
 #[derive(Copy, Clone, Hash, Debug, PartialEq, Eq, PartialOrd, Ord, Default, clap::ValueEnum)]
 pub enum OutputFormat {
-    /// Print diagnostics verbosely, with context and helpful hints \[default\].
+    /// Print diagnostics verbosely, with context and helpful hints (default).
     ///
     /// Diagnostic messages may include additional context and
     /// annotations on the input to help understand the message.
@@ -321,6 +321,9 @@ pub enum OutputFormat {
     /// dropped.
     #[value(name = "concise")]
     Concise,
+    /// Print diagnostics in the JSON format expected by GitLab Code Quality reports.
+    #[value(name = "gitlab")]
+    Gitlab,
 }
 
 impl From<OutputFormat> for ty_project::metadata::options::OutputFormat {
@@ -328,6 +331,7 @@ impl From<OutputFormat> for ty_project::metadata::options::OutputFormat {
         match format {
             OutputFormat::Full => Self::Full,
             OutputFormat::Concise => Self::Concise,
+            OutputFormat::Gitlab => Self::Gitlab,
         }
     }
 }

crates/ty/src/lib.rs

@@ -21,7 +21,7 @@ use clap::{CommandFactory, Parser};
 use colored::Colorize;
 use crossbeam::channel as crossbeam_channel;
 use rayon::ThreadPoolBuilder;
-use ruff_db::diagnostic::{Diagnostic, DisplayDiagnosticConfig, Severity};
+use ruff_db::diagnostic::{Diagnostic, DisplayDiagnosticConfig, DisplayDiagnostics, Severity};
 use ruff_db::files::File;
 use ruff_db::max_parallelism;
 use ruff_db::system::{OsSystem, SystemPath, SystemPathBuf};
@@ -319,37 +319,48 @@ impl MainLoop {
                             return Ok(ExitStatus::Success);
                         }
 
+                        let is_human_readable = terminal_settings.output_format.is_human_readable();
+
                         if result.is_empty() {
-                            writeln!(
-                                self.printer.stream_for_success_summary(),
-                                "{}",
-                                "All checks passed!".green().bold()
-                            )?;
+                            if is_human_readable {
+                                writeln!(
+                                    self.printer.stream_for_success_summary(),
+                                    "{}",
+                                    "All checks passed!".green().bold()
+                                )?;
+                            }
 
                             if self.watcher.is_none() {
                                 return Ok(ExitStatus::Success);
                             }
                         } else {
-                            let mut max_severity = Severity::Info;
                             let diagnostics_count = result.len();
 
                             let mut stdout = self.printer.stream_for_details().lock();
-                            for diagnostic in result {
-                                // Only render diagnostics if they're going to be displayed, since doing
-                                // so is expensive.
-                                if stdout.is_enabled() {
-                                    write!(stdout, "{}", diagnostic.display(db, &display_config))?;
-                                }
+                            let max_severity = result
+                                .iter()
+                                .map(Diagnostic::severity)
+                                .max()
+                                .unwrap_or(Severity::Info);
 
-                                max_severity = max_severity.max(diagnostic.severity());
+                            // Only render diagnostics if they're going to be displayed, since doing
+                            // so is expensive.
+                            if stdout.is_enabled() {
+                                write!(
+                                    stdout,
+                                    "{}",
+                                    DisplayDiagnostics::new(db, &display_config, &result)
+                                )?;
                             }
 
-                            writeln!(
-                                self.printer.stream_for_failure_summary(),
-                                "Found {} diagnostic{}",
-                                diagnostics_count,
-                                if diagnostics_count > 1 { "s" } else { "" }
-                            )?;
+                            if is_human_readable {
+                                writeln!(
+                                    self.printer.stream_for_failure_summary(),
+                                    "Found {} diagnostic{}",
+                                    diagnostics_count,
+                                    if diagnostics_count > 1 { "s" } else { "" }
+                                )?;
+                            }
 
                             if max_severity.is_fatal() {
                                 tracing::warn!(