refactor: add prompt_id as hidden type, fix imports, add docstrings

- Add PROMPT_ID as a new hidden type in the Hidden enum, HiddenHolder,
  HiddenInputTypeDict, and execution engine resolution (both V3 and legacy)
- Refactor GetImageSize to use cls.hidden.prompt_id instead of manually
  calling get_executing_context() — addresses reviewer feedback
- Remove lazy import of get_executing_context from nodes_images.py
- Add docstrings to send_progress_text, _display_text, HiddenHolder,
  and HiddenHolder.from_dict

Amp-Thread-ID: https://ampcode.com/threads/T-019ca1cb-0150-7549-8b1b-6713060d3408

comfy/comfy_types/node_typing.py

@@ -193,6 +193,8 @@ class HiddenInputTypeDict(TypedDict):
     """EXTRA_PNGINFO is a dictionary that will be copied into the metadata of any .png files saved. Custom nodes can store additional information in this dictionary for saving (or as a way to communicate with a downstream node)."""
     dynprompt: NotRequired[Literal["DYNPROMPT"]]
     """DYNPROMPT is an instance of comfy_execution.graph.DynamicPrompt. It differs from PROMPT in that it may mutate during the course of execution in response to Node Expansion."""
+    prompt_id: NotRequired[Literal["PROMPT_ID"]]
+    """PROMPT_ID is the unique identifier of the current prompt/job being executed. Useful for associating progress updates with specific jobs."""
 
 
 class InputTypeDict(TypedDict):