Add e2e snapshot test for ctrl-c 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 for e2e snapshot
tests, fixes `expect_milestone` to not drop unmatched milestones, and strips
`^C` terminal echo in redaction for cross-platform consistency.

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

Author: branchseer

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/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.