Fix key-loop in Tree.add to reuse existing children for literal segments after a matcher boundary

The key-loop unconditionally called self.parse() and overwrote
result.children[key] for literal segments after the walk-down loop
broke at a parameter. This destroyed pre-existing subtrees (with their
method children and handlers) when a second route (e.g. POST) shared
the same literal sub-segments after a parameter.

Added a children.get(key) reuse check before parse(), mirroring the
walk-down loop's child-reuse logic but extended past the boundary
where parameters live in path_matchers, not children.

packages/kaya-core/src/kaya/core/_tree.py

@@ -97,6 +97,11 @@ class Tree:
                 result = existing
                 key = next(it, None)
                 continue
+            child = result.children.get(key)
+            if child is not None:
+                result = child
+                key = next(it, None)
+                continue
             new_node = self.parse(key, result)
             if isinstance(new_node, Node):
                 result.children[key] = new_node

packages/kaya-core/tests/test_asgi.py

@@ -216,3 +216,37 @@ class AsgiTest(unittest.TestCase):
             r = await client.get("/restaurants/42/unknown")
             self.assertEqual(404, r.status_code)
 
+    @async_test
+    async def test_nested_param_routes_multiple_methods(self):
+        app = KayaApp()
+
+        @app.GET('/restaurants/${id}')
+        async def restaurant(ctx: HttpContext, id: str) -> None:
+            await ctx.send_str(200, f"restaurant:{id}")
+
+        @app.GET('/restaurants/${id}/menu')
+        async def menu_get(ctx: HttpContext, id: str) -> None:
+            await ctx.send_str(200, f"menu_get:{id}")
+
+        @app.POST('/restaurants/${id}/menu')
+        async def menu_post(ctx: HttpContext, id: str) -> None:
+            await ctx.send_str(200, f"menu_post:{id}")
+
+        transport = httpx.ASGITransport(app=app)
+
+        async with httpx.AsyncClient(transport=transport, base_url="http://127.0.0.1:80") as client:
+            r = await client.get("/restaurants/42")
+            self.assertEqual(200, r.status_code)
+            self.assertEqual("restaurant:42", r.text)
+
+            r = await client.get("/restaurants/42/menu")
+            self.assertEqual(200, r.status_code)
+            self.assertEqual("menu_get:42", r.text)
+
+            r = await client.post("/restaurants/42/menu")
+            self.assertEqual(200, r.status_code)
+            self.assertEqual("menu_post:42", r.text)
+
+            r = await client.put("/restaurants/42/menu")
+            self.assertEqual(404, r.status_code)
+

packages/kaya-core/tests/test_tree.py

@@ -156,3 +156,33 @@ class TreeTest(unittest.TestCase):
         with self.assertRaises(ValueError):
             tree.add((p for p in ('a', '${id:int}', 'x')), HttpMethod.GET, self.handlers[1])
 
+    def test_nested_routes_different_methods_share_subtree(self):
+        tree = Tree()
+        tree.add((p for p in ('restaurants', '${id}', 'menu')), HttpMethod.GET, self.handlers[0])
+        tree.add((p for p in ('restaurants', '${id}', 'menu')), HttpMethod.POST, self.handlers[1])
+        h0 = Maybe.of_nullable(tree.get_handler('/restaurants/42/menu', HttpMethod.GET)).map(lambda it: it[0]).or_none()
+        h1 = Maybe.of_nullable(tree.get_handler('/restaurants/42/menu', HttpMethod.POST)).map(lambda it: it[0]).or_none()
+        self.assertIs(self.handlers[0], h0)
+        self.assertIs(self.handlers[1], h1)
+
+    def test_nested_routes_different_methods_reverse_order(self):
+        tree = Tree()
+        tree.add((p for p in ('restaurants', '${id}', 'menu')), HttpMethod.POST, self.handlers[1])
+        tree.add((p for p in ('restaurants', '${id}', 'menu')), HttpMethod.GET, self.handlers[0])
+        h0 = Maybe.of_nullable(tree.get_handler('/restaurants/42/menu', HttpMethod.GET)).map(lambda it: it[0]).or_none()
+        h1 = Maybe.of_nullable(tree.get_handler('/restaurants/42/menu', HttpMethod.POST)).map(lambda it: it[0]).or_none()
+        self.assertIs(self.handlers[0], h0)
+        self.assertIs(self.handlers[1], h1)
+
+    def test_nested_routes_combined_detail_and_menu_methods(self):
+        tree = Tree()
+        tree.add((p for p in ('restaurants', '${id}')), HttpMethod.GET, self.handlers[0])
+        tree.add((p for p in ('restaurants', '${id}', 'menu')), HttpMethod.GET, self.handlers[1])
+        tree.add((p for p in ('restaurants', '${id}', 'menu')), HttpMethod.POST, self.handlers[2])
+        h0 = Maybe.of_nullable(tree.get_handler('/restaurants/42', HttpMethod.GET)).map(lambda it: it[0]).or_none()
+        h1 = Maybe.of_nullable(tree.get_handler('/restaurants/42/menu', HttpMethod.GET)).map(lambda it: it[0]).or_none()
+        h2 = Maybe.of_nullable(tree.get_handler('/restaurants/42/menu', HttpMethod.POST)).map(lambda it: it[0]).or_none()
+        self.assertIs(self.handlers[0], h0)
+        self.assertIs(self.handlers[1], h1)
+        self.assertIs(self.handlers[2], h2)
+