Studio: prompt variables into prompt editor (#6434)

* add custom and system variable feature in system prompt

* missing function use

* feat: prompt variables editor ux

Co-authored-by: CodeMan62 <175127021+CodeMan62@users.noreply.github.com>

* fix: guard prompt variable defaults

* refine prompt variables editor layout

* fix: harden prompt variable substitution

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Refine prompt variables editor copy and built-in token labels

---------

Co-authored-by: CodeMan62 <sharmahimanshu15082007@gmail.com>
Co-authored-by: CodeMan62 <175127021+CodeMan62@users.noreply.github.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Leo Borcherding <borchborchmail@gmail.com>

tests/studio/test_chat_prompt_variables.py

@@ -0,0 +1,42 @@
+"""Regression checks for system prompt variable substitution."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+
+WORKSPACE = Path(__file__).resolve().parents[2]
+ADAPTER_SRC = (WORKSPACE / "studio/frontend/src/features/chat/api/chat-adapter.ts").read_text()
+
+
+def _function_source(name: str) -> str:
+    start = ADAPTER_SRC.index(f"function {name}")
+    body_start = ADAPTER_SRC.index("{", start)
+    depth = 0
+    for index in range(body_start, len(ADAPTER_SRC)):
+        if ADAPTER_SRC[index] == "{":
+            depth += 1
+        elif ADAPTER_SRC[index] == "}":
+            depth -= 1
+            if depth == 0:
+                return ADAPTER_SRC[start : index + 1]
+    raise AssertionError(f"Could not parse function body for {name}")
+
+
+def test_prompt_variable_builtins_use_local_time_helpers():
+    resolver = _function_source("resolveSystemPromptVariables")
+    assert "formatLocalDate(now)" in resolver
+    assert "formatLocalTime(now)" in resolver
+    assert "formatTimezoneOffset(now)" in resolver
+    assert "toISOString()" not in resolver
+
+
+def test_prompt_variable_builtins_use_own_property_lookup():
+    resolver = _function_source("resolveSystemPromptVariables")
+    assert "if (hasOwn(systemVariables, key))" in resolver
+    assert "key in systemVariables" not in resolver
+
+
+def test_prompt_variable_nested_lookup_ignores_prototype_properties():
+    nested_lookup = _function_source("getNestedValue")
+    assert "if (!hasOwn(current, part))" in nested_lookup