fix: support encoding of special characters in path params for gen2 f…

[IzaakGough]

Fixes firebase/firebase-functions#1459

Node.js decodes HTTP header bytes as Latin-1 by default. When Firestore (and potentially other services) send non-ASCII document paths as raw UTF-8 bytes in binary-mode CloudEvent extension attributes, getBinaryCloudEventContext receives them already corrupted — e.g. Helvétios arrives as Helvétios.

This is the root cause of firebase/firebase-functions#1459, which has been open since 2023 and affects all Gen2 Firestore triggers with non-ASCII document IDs.

Fix

Re-decode each header value from binary to UTF-8 before setting it on the CloudEvent:

const val = req.header(name);
context[attributeName] = val ? Buffer.from(val, 'binary').toString('utf-8') : val;

ASCII values are unaffected (ASCII is identical in both encodings).