fix(webhook): raise InvalidWebhookError for corrupt gzip streams

[RaphaelFakhri]

Why

gunzip_payload converts gzip failures into InvalidWebhookError, but it only catches gzip.BadGzipFile, OSError, and EOFError. A body that starts with the gzip magic bytes and has a damaged deflate stream makes gzip.decompress raise zlib.error, which is none of those. The raw zlib.error escapes from gunzip_payload, verify_and_parse_webhook, parse_sqs, and parse_sns, so a handler that catches InvalidWebhookError to reject bad requests returns a server error instead.

Changes

• Catch zlib.error in gunzip_payload and raise InvalidWebhookError(INVALID_WEBHOOK_GZIP_FAILED).
• Add a test that corrupts the deflate stream of a valid gzip body.

The new test fails on master with zlib.error and passes with the change.