Add ctrl-c e2e test for SIGINT propagation to running tasks

Add a `vtt exit-on-ctrlc` subcommand and e2e fixture that verifies SIGINT
propagates to concurrent tasks when the user presses Ctrl+C. The test runs
two packages with `vt run -r dev`, synchronizes via milestone protocol,
then sends ctrl-c and verifies both tasks receive and handle it.

Also adds `ctrl-c` as a new `write-key` interaction type, fixes
`expect_milestone` to preserve unmatched milestones, strips `^C`
terminal echo in redaction, and normalizes Windows CTRL_C exit code (512)
to match Unix (1) for cross-platform snapshot consistency.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: branchseer

crates/vite_task/src/session/execute/mod.rs

@@ -560,6 +560,7 @@ async fn spawn_inherited(
                     }
                 }
             }
+
             Ok(())
         });
     }

crates/vite_task_bin/tests/e2e_snapshots/fixtures/ctrl-c/snapshots/ctrl-c terminates running tasks.snap

@@ -12,4 +12,4 @@ expression: e2e_outputs
 @ write-key: ctrl-c
 ~/packages/a$ vtt exit-on-ctrlc ⊘ cache disabled
 ~/packages/b$ vtt exit-on-ctrlc ⊘ cache disabled
-^Cctrl-c receivedctrl-c received
+ctrl-c receivedctrl-c received

crates/vite_task_bin/tests/e2e_snapshots/main.rs

@@ -386,7 +386,9 @@ fn run_case_inner(tmpdir: &AbsolutePath, fixture_path: &std::path::Path, fixture
                     e2e_outputs.push_str("[timeout]");
                 }
                 TerminationState::Exited(exit_code) => {
-                    if *exit_code != 0 {
+                    // Normalize Windows CTRL_C exit code (512) to match Unix (1).
+                    let exit_code = if cfg!(windows) && *exit_code == 512 { 1 } else { *exit_code };
+                    if exit_code != 0 {
                         e2e_outputs.push_str(vite_str::format!("[{exit_code}]").as_str());
                     }
                 }

crates/vite_task_bin/tests/e2e_snapshots/redact.rs

@@ -101,6 +101,15 @@ pub fn redact_e2e_output(mut output: String, workspace_root: &str) -> String {
     let mise_warning_regex = regex::Regex::new(r"(?m)^mise WARN\s+.*\n?").unwrap();
     output = mise_warning_regex.replace_all(&output, "").into_owned();
 
+    // Remove ^C echo that Unix terminal drivers emit when ETX (0x03) is written
+    // to the PTY. Windows ConPTY does not echo it.
+    {
+        use cow_utils::CowUtils as _;
+        if let Cow::Owned(replaced) = output.as_str().cow_replace("^C", "") {
+            output = replaced;
+        }
+    }
+
     // Sort consecutive diagnostic blocks to handle non-deterministic tool output
     // (e.g., oxlint reports warnings in arbitrary order due to multi-threading).
     // Each block starts with "  ! " and ends at the next empty line.