Save history on SIGTERM and SIGHUP before exit

Previously, SIGTERM immediately re-raised with SIG_DFL, killing
fish without saving history. SIGHUP deferred via a flag but never
re-raised, so the parent saw a normal exit instead of signal death.

Unify both signals: the handler stores the signal number in a single
AtomicI32, the reader loop exits normally, throwing_main() saves
history and re-raises with SIG_DFL so the parent sees WIFSIGNALED.

Fixes #10300

Closes #12615

tests/pexpects/sigterm_history.py

@@ -0,0 +1,50 @@
+#!/usr/bin/env python3
+from pexpect_helper import SpawnedProc
+import os
+import pexpect
+import signal
+import sys
+
+sp = SpawnedProc()
+fish_pid = sp.spawn.pid
+assert fish_pid is not None
+sp.expect_prompt()
+
+# Set up a fish_exit handler that prints a marker to stdout.
+sp.sendline("function __test_exit --on-event fish_exit; echo SIGTERM_CLEANUP_OK; end")
+sp.expect_prompt()
+
+# Run a command to populate history.
+sp.sendline("echo sigterm_history_test_%d" % fish_pid)
+sp.expect_prompt()
+
+# Send SIGTERM.
+os.kill(fish_pid, signal.SIGTERM)
+
+# Try to catch the cleanup marker before EOF.
+idx = sp.spawn.expect(["SIGTERM_CLEANUP_OK", pexpect.EOF], timeout=5)
+if idx == 1:
+    print("FAIL: fish_exit did not fire on SIGTERM (no cleanup marker in output)")
+    sys.exit(1)
+
+# Drain remaining output.
+sp.spawn.expect(pexpect.EOF, timeout=5)
+sp.spawn.close()
+
+# Verify death was by signal (re-raised SIGTERM), not normal exit.
+if sp.spawn.signalstatus != signal.SIGTERM:
+    print(
+        "FAIL: expected SIGTERM, got signal=%s exit=%s"
+        % (sp.spawn.signalstatus, sp.spawn.exitstatus)
+    )
+    sys.exit(1)
+
+# Check history file for NUL bytes.
+histfile = os.path.join(os.environ["XDG_DATA_HOME"], "fish", "fish_history")
+with open(histfile, "rb") as f:
+    raw = f.read()
+assert len(raw) != 0, "history file is empty"
+nul_count = raw.count(b"\x00")
+if nul_count > 0:
+    print("FAIL: history file contains %d NUL bytes" % nul_count)
+    sys.exit(1)