Is there an idiomatic expression to refer to an “ambush” journalist?

The contemporary meaning of 'predatory journalist' perhaps comes closest to the contemporary meaning of 'ambush journalist'. Another, possibly better candidate is 'stalkarazzo' (in plural, 'stalkarazzi', modeled on 'paparazzi' and 'paparazzo').

In the Newspapers+ Publishers Extra corpus (paywalled), the uses (including repetitions and false positives; plurals of 'reporter' and 'journalist' are also included in count totals) of various terms break down thus:

Ambush reporter/journalist:       104, 1917-2006 / 36, 1981-2014
Aggressive reporter/journalist:  5374, 1884-2018 / 1127, 1870-2018
Predatory reporter/journalist:     54, 1904-2014 / 71, 1907-2009
Stalkarazzi:                      368, 1994-2014

To be sure, in use each of the terms has unique nuances and a unique range of meaning. 'Ambush journalist', for example, is not so broad a term as 'aggressive journalist' and, historically at least, 'predatory journalist' was also used to label journalists who stole their material.

The meaning of 'stalkarazzi' shades more toward a label for journalists who use modern surveillance and other technologies (long-range photography, shotgun microphones, camera phones) than does 'ambush journalists', although both terms include the full range of the other: 'ambush journalists' describes the tactics and strategies of 'stalkarazzi' and vice versa.

A contemporary example of the use of 'stalkarazzi' from The Los Angeles Times (Los Angeles, California), 18 May 2014:

THE MARTINEZ. The last of the string of plush hotels that line the Croisette and another popular spot for the celebrities and the people who stalk them...it was here last year that the notorious Psy impersonator was spotted...waving to the stalkarazzi...the camera-phone crowds waiting outside the barriers snapped pictures of him and believed he was the real...deal.

An example using both 'stalkarazzi' and 'predatory journalists' from The Los Angeles Times, 16 Mar 1998. As implied, 'predatory journalists' is the broader category that, like 'aggressive journalists', includes both 'stalkarazzi' and 'ambush journalists':

Unfortunately, the problems associated with the stalkarazzi are not limited to California; these predatory journalists often cross state lines to gain pictures and recordings.

In both contemporary and historical use, 'aggressive reporter' covers a much broader range of meaning than 'ambush reporter', and is more frequently used with a neutral or positive connotation. This example comes from The Morning Call (Allentown, Pennsylvania), 15 Nov 2018:

"The White House has made very clear that they don't like the content of the reporting by CNN and Jim Acosta," Boutrous said. "Rudeness really is a code word for 'I don't like you being an aggressive reporter.'"

I have found ambush reporter more commonly than ambush journalist in my searches. For instance, "ambush reporter" appeared a few times in Dilbert, as Dogbert was an "ambush journalist" who acts like "Mike Wallace." These examples are from the early 1990s, and reflect the term's wide application to everyone from 60 Minutes's Mike Wallace to the reporters who would approach sports stars or CEOs on the street.

Extended articles on the subject like this one or this one don't vary much from "ambush reporter" or "reporter." You might also be able to use variants like ambush interviewer. This suggests to me that there isn't a precise idiom available to describe this person beyond applying the "ambush" or similar label to reporter, journalist, or interviewer.