Add prompt_id to progress_text binary WS messages

Add supports_progress_text_metadata feature flag and extend
send_progress_text() to accept optional prompt_id param. When
prompt_id is provided and the client supports the new format,
the binary wire format includes a length-prefixed prompt_id field:

  [4B event_type][4B prompt_id_len][prompt_id][4B node_id_len][node_id][text]

Legacy format preserved for clients without the flag.

Both callers (nodes_images.py, client.py) updated to pass prompt_id
from get_executing_context().

Part of COM-12671: parallel workflow execution support.

Amp-Thread-ID: https://ampcode.com/threads/T-019c79f7-f19b-70d9-b662-0687cc206282

comfy_api/feature_flags.py

@@ -12,6 +12,7 @@ from comfy.cli_args import args
 # Default server capabilities
 SERVER_FEATURE_FLAGS: dict[str, Any] = {
     "supports_preview_metadata": True,
+    "supports_progress_text_metadata": True,
     "max_upload_size": args.max_upload_size * 1024 * 1024, # Convert MB to bytes
     "extension": {"manager": {"supports_v4": True}},
     "node_replacements": True,