`cell_hit_map` in MGPD and SiTracker digi loops with copy, not ref

[wdconinc]

for (auto item : cell_hit_map) copies each entry (including the MutableRawTrackerHit) unnecessarily. Iterating by const& (ideally with structured bindings) avoids extra copies and makes the intent clearer.

Originally posted by @Copilot in #2410 (comment)