FIX: Ensure valid json

[PatMulligan]

Fix: construct EVENT message with json.dumps instead of string interpolation

Why

The old implementation built JSON-like strings by interpolation. This works for simple inputs, but as soon as values contain quotes, backslashes, or Unicode characters, the result is not valid JSON and will be rejected by relays.

Change

- event_to_forward = f"""["EVENT", "{s_original}", {event_json}]"""
+ event_to_forward = json.dumps(["EVENT", s_original, json.loads(event_json)])

Example of the problem

Inputs

import json

s_original = 'sub"one'
event_json = '{"content": "hello \\"nostr\\"", "tags": []}'

Old code

event_to_forward = f"""["EVENT", "{s_original}", {event_json}]"""
print(event_to_forward)
# ["EVENT", "sub"one", {"content": "hello \"nostr\"", "tags": []}]
json.loads(event_to_forward)
# raises json.decoder.JSONDecodeError

The string looks like JSON but cannot be parsed.

New code

event_to_forward = json.dumps(["EVENT", s_original, json.loads(event_json)])
print(event_to_forward)
# ["EVENT", "sub\"one", {"content": "hello \"nostr\"", "tags": []}]
json.loads(event_to_forward)  # ✅ works

Benefits

• Always produces valid JSON.
• Correctly escapes special characters in subscription IDs and event data.
• Removes risk of malformed payloads and relay rejections.
• Easier to maintain and extend (all inputs handled as objects, not string fragments).