test(nodes): add unit tests for prompt template engine and polish

Adds 57 unit tests covering the custom template engine (stdlib-only, no
Jinja2) in nodes/test/test_prompt_template_engine.py — variable
substitution, dotted paths, missing variables, builtins, context-over-
builtin overrides, if/elif/else, nested blocks, for loops, escape
sequences, malformed/unclosed tags, HTML pass-through, and ReDoS /
recursion safety. The test module loads template_engine.py directly via
importlib so it does not trigger the depends-based package init.

Also addresses outstanding review feedback on PR #573:
- Mark the node as experimental in services.json (per asclearuc)
- Add an explicit {{question}} substitution test case in services.json
  (per CodeRabbit)
- Add missing return type annotations on _resolve (Any), _at_top_level
  (bool), _handle_if (int) and _handle_for (int) to satisfy Ruff ANN202

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

Author: charliegillet

nodes/src/nodes/prompt_template/services.json

@@ -21,7 +21,7 @@
 	//		Capabilities are flags that change the behavior of the underlying
 	//		engine
 	//
-	"capabilities": [],
+	"capabilities": ["experimental"],
 	//
 	// Optional:
 	//		Register is either filter, endpoint or ignored if not specified. If the
@@ -142,6 +142,20 @@
 						"contains": "Summarize the following:"
 					}
 				}
+			},
+			{
+				"config": {
+					"template": "Q={{question}} | I={{input}}",
+					"variables": {"input": "ctx"}
+				},
+				"questions": {
+					"questions": [{"text": "what is this?"}]
+				},
+				"expect": {
+					"questions": {
+						"contains": "Q=what is this?"
+					}
+				}
 			}
 		]
 	},

nodes/src/nodes/prompt_template/template_engine.py

@@ -37,6 +37,7 @@
 import uuid as _uuid
 from datetime import datetime, timezone
 from random import random as _random
+from typing import Any
 
 
 # Sentinel used to distinguish "key missing" from "key is None"
@@ -64,7 +65,7 @@
 # ---------------------------------------------------------------------------
 # Context value resolution
 # ---------------------------------------------------------------------------
-def _resolve(name: str, context: dict):
+def _resolve(name: str, context: dict) -> Any:
     """Resolve a dotted name against *context*, falling back to built-ins.
 
     Resolution order: context takes priority over built-in helpers.
@@ -169,7 +170,7 @@ def _eval_tokens(tokens: list[tuple[str, str]], context: dict) -> str:
     return ''.join(parts)
 
 
-def _at_top_level(tokens, start, pos):
+def _at_top_level(tokens: list[tuple[str, str]], start: int, pos: int) -> bool:
     """Check if *pos* is at the top nesting level between *start* and *pos*."""
     depth_if = 0
     depth_for = 0
@@ -188,7 +189,12 @@ def _at_top_level(tokens, start, pos):
     return depth_if == 0 and depth_for == 0
 
 
-def _handle_if(tokens, start, context, parts):
+def _handle_if(
+    tokens: list[tuple[str, str]],
+    start: int,
+    context: dict,
+    parts: list[str],
+) -> int:
     """Process an {% if %}...{% elif %}...{% else %}...{% endif %} block."""
     end = _find_matching_end(tokens, start + 1, 'if', 'endif')
     if end == -1:
@@ -225,7 +231,12 @@ def _handle_if(tokens, start, context, parts):
     return end + 1
 
 
-def _handle_for(tokens, start, context, parts):
+def _handle_for(
+    tokens: list[tuple[str, str]],
+    start: int,
+    context: dict,
+    parts: list[str],
+) -> int:
     """Process a {% for item in list %}...{% endfor %} block."""
     end = _find_matching_end(tokens, start + 1, 'for', 'endfor')
     if end == -1: