Improve venv shell activation in fish shell (#8804)

Instead of mimicking a user typing the command we use fish's builtin `--init-command` option.

src/poetry/utils/shell.py

@@ -106,7 +106,12 @@ def activate(self, env: VirtualEnv) -> int | None:
         cmd = f"{self._get_source_command()} {shlex.quote(str(activate_path))}"
 
         with env.temp_environ():
-            args = ["-e", cmd] if self._name == "nu" else ["-i"]
+            if self._name == "nu":
+                args = ["-e", cmd]
+            elif self._name == "fish":
+                args = ["-i", "--init-command", cmd]
+            else:
+                args = ["-i"]
 
             c = pexpect.spawn(
                 self._path, args, dimensions=(terminal.lines, terminal.columns)
@@ -120,14 +125,11 @@ def activate(self, env: VirtualEnv) -> int | None:
             c.sendline(f"emulate bash -c '. {shlex.quote(str(activate_path))}'")
         elif self._name == "xonsh":
             c.sendline(f"vox activate {shlex.quote(str(env.path))}")
-        elif self._name == "nu":
-            # If this is nu, we don't want to send the activation command to the
+        elif self._name in ["nu", "fish"]:
+            # If this is nu or fish, we don't want to send the activation command to the
             # command line since we already ran it via the shell's invocation.
             pass
         else:
-            if self._name in ["fish"]:
-                # Under fish, "\r" should be sent explicitly
-                cmd += "\r"
             c.sendline(cmd)
 
         def resize(sig: Any, data: Any) -> None: