Respect `--output-format` with `--watch` (#21097)

Summary
--

Fixes #19550

This PR copies our non-watch diagnostic rendering code into
`Printer::write_continuously` in preview mode, allowing it to use
whatever output format is passed in.

I initially marked this as also fixing #19552, but I guess that's not
true currently but will be true once this is stabilized and we can
remove the warning.

Test Plan
--

Existing tests, but I don't think we have any `watch` tests, so some
manual testing as well. The default with just `ruff check --watch` is
still `concise`, adding just `--preview` still gives the `full` output,
and then specifying any other output format works, with JSON as one
example:

<img width="695" height="719" alt="Screenshot 2025-10-27 at 9 21 41 AM"
src="https://github.com/user-attachments/assets/98957911-d216-4fc4-8b6c-22c56c963b3f"
/>

crates/ruff/src/printer.rs

@@ -9,9 +9,7 @@ use itertools::{Itertools, iterate};
 use ruff_linter::linter::FixTable;
 use serde::Serialize;
 
-use ruff_db::diagnostic::{
-    Diagnostic, DiagnosticFormat, DisplayDiagnosticConfig, DisplayDiagnostics, SecondaryCode,
-};
+use ruff_db::diagnostic::{Diagnostic, DisplayDiagnosticConfig, SecondaryCode};
 use ruff_linter::fs::relativize_path;
 use ruff_linter::logging::LogLevel;
 use ruff_linter::message::{EmitterContext, render_diagnostics};
@@ -390,21 +388,18 @@ impl Printer {
 
             let context = EmitterContext::new(&diagnostics.notebook_indexes);
             let format = if preview {
-                DiagnosticFormat::Full
+                self.format
             } else {
-                DiagnosticFormat::Concise
+                OutputFormat::Concise
             };
             let config = DisplayDiagnosticConfig::default()
+                .preview(preview)
                 .hide_severity(true)
                 .color(!cfg!(test) && colored::control::SHOULD_COLORIZE.should_colorize())
                 .with_show_fix_status(show_fix_status(self.fix_mode, fixables.as_ref()))
-                .format(format)
-                .with_fix_applicability(self.unsafe_fixes.required_applicability());
-            write!(
-                writer,
-                "{}",
-                DisplayDiagnostics::new(&context, &config, &diagnostics.inner)
-            )?;
+                .with_fix_applicability(self.unsafe_fixes.required_applicability())
+                .show_fix_diff(preview);
+            render_diagnostics(writer, format, config, &context, &diagnostics.inner)?;
         }
         writer.flush()?;