feat: Add JSON parsers infrastructure (Phase 1)

- Create json_utils.py with parse_json_dict() and parse_json_list() functions
- Add comprehensive unit tests (49 tests, all passing)
- Add deprecation warnings to string_utils.py for pipe-delimited parsing
- Follows PRD Phase 1: Parser Infrastructure

This is the first step in replacing pipe-delimited LLM output format with
structured JSON format as per PRD_STANDARDIZE_LIST_FORMATS.md

src/utils/string_utils.py

@@ -35,6 +35,12 @@ def extract_text_from_delimiters(
 
     Returns:
         - str: The extracted answer or "N/A" if no match is found.
+
+    .. deprecated:: 2026.02
+        Using Delimiter.PIPE with this function is deprecated and will be removed
+        in a future version. Prompts should return JSON format instead of
+        pipe-delimited strings. Use `json_utils.parse_json_list()` or
+        `json_utils.parse_json_dict()` for parsing JSON responses.
     """
     if not isinstance(raw_output, str):
         raise TypeError(
@@ -50,6 +56,13 @@ def extract_text_from_delimiters(
 
     # Define a pattern based on the delimiter type
     if delimiter == Delimiter.PIPE:
+        warnings.warn(
+            "Delimiter.PIPE is deprecated for LLM output parsing. "
+            "Prompts should return JSON format instead. "
+            "Use json_utils.parse_json_list() or json_utils.parse_json_dict() instead.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
         pattern = PIPE_PATTERN
     elif delimiter == Delimiter.BACKTICK:
         pattern = BACKTICK_PATTERN
@@ -191,7 +204,20 @@ def universal_json_load(s: str):
     Extract and parse all highest-level valid JSON objects from a string.
     Returns the last valid top-level JSON object found (dict, list, etc).
     Raises ValueError if no valid JSON objects are found. Does not clean up syntax.
+
+    .. deprecated:: 2026.02
+        This function is deprecated and will be removed in a future version.
+        Use `json_utils.parse_json_dict()` for dictionary outputs or
+        `json_utils.parse_json_list()` for list outputs instead.
+        The new parsers provide type-specific parsing with better error handling.
     """
+    warnings.warn(
+        "universal_json_load is deprecated and will be removed in a future version. "
+        "Use json_utils.parse_json_dict() or json_utils.parse_json_list() instead.",
+        DeprecationWarning,
+        stacklevel=2,
+    )
+
     if not isinstance(s, str):
         raise TypeError(f"Expected string input, got {type(s).__name__}")
     found = []