docs: add promptfirewall callback integration to integrations listing

Description

Request to add promptfirewall to the callbacks integration listing.

llama-index-callbacks-promptfirewall is an independently published PyPI package that provides PII detection and prompt injection blocking as both a LlamaIndex callback handler (legacy) and an instrumentation event handler (new).

This is a listing request only — no code is being added to the monorepo.

Package Details

What It Does

Sub-millisecond PII detection and prompt injection firewall for LlamaIndex applications. Written in Rust, exposed to Python via PyO3.

Two handler modes:

1. Legacy: PromptFirewallHandler(BaseCallbackHandler) — hooks CBEventType.LLM and CBEventType.QUERY
2. New: PromptFirewallEventHandler(BaseEventHandler) — handles LLMChatStartEvent, LLMCompletionStartEvent, LLMPredictStartEvent

Supported PII types: SSN, credit card (Luhn-validated), IBAN (ISO 7064), email, phone, IP address, AWS keys, API keys

Quick Start

from llama_index_callbacks_promptfirewall import PromptFirewallHandler
from llama_index.core.callbacks import CallbackManager
from llama_index.core import Settings

handler = PromptFirewallHandler()
Settings.callback_manager = CallbackManager([handler])
# All LLM calls and queries are now scanned

Performance

The only comparable integration is llama-index-postprocessor-presidio (PII only, ~180ms, no injection detection).