Fix "Long running tasks" logging when dynamic_ui is disabled

[lowvoltage]

This addresses #23603

Problem

In Pants v2.32, when dynamic_ui = false is configured (the default on CI where no TTY is present), Pants would periodically print Long running tasks: lines to the console. This output serves as a heartbeat that prevents CI systems from timing out long-running builds.

In v2.33, this console output no longer appears, causing CI timeout failures.

Root Cause

Commit 52d5fa0 (#23485, "Skip queueing workunit messages for consumers that never poll to avoid unbound memory growth") introduced a regression.

The WorkunitStore constructor gained a ui_consumers flag that controls whether messages are sent to the heavy_hitters_data channel:

// session.rs line 164
let workunit_store = WorkunitStore::new(!dynamic_ui, max_workunit_level, false, dynamic_ui);
//                                                                              ^^^^^^^^^^
//                                                                              ui_consumers

When dynamic_ui = false, ui_consumers = false, so self.ui_sender = None in the WorkunitStore. This means workunit StoreMsg messages (Started/Completed/Canceled) are never sent to the heavy_hitters_data receiver channel.

When the SessionDisplay::Logging variant calls straggling_workunits() to print "Long running tasks:", HeavyHittersData::refresh_store() finds nothing because no messages were ever delivered to its receiver — the sender was None.

The incorrect assumption

The comment in the original commit says: "the heavy hitters consumer only exists for the dynamic UI". This is incorrect:

• SessionDisplay::ConsoleUI (dynamic UI on) → uses heavy_hitters() to render the dynamic task display
• SessionDisplay::Logging (dynamic UI off) → uses straggling_workunits() to log long-running tasks

Both read from the same heavy_hitters_data channel. Both need messages delivered to function.

Fix

Always enable the heavy hitters channel (ui_consumers = true), since it is needed by both ConsoleUI (via heavy_hitters()) and Logging (via straggling_workunits()).

-let workunit_store = WorkunitStore::new(!dynamic_ui, max_workunit_level, false, dynamic_ui);
+let workunit_store = WorkunitStore::new(!dynamic_ui, max_workunit_level, false, true);

This is a one-line change in src/rust/engine/src/session.rs.

Impact on the original optimization

The original PR aimed to avoid unbounded memory growth from messages queued for consumers that never poll. This fix means the heavy hitters channel will always have messages sent to it, but this is safe because:

• When dynamic_ui = true: ConsoleUI::render() polls heavy_hitters() regularly
• When dynamic_ui = false: SessionDisplay::Logging polls straggling_workunits() regularly

In both cases, the channel is actively polled, so messages do not accumulate.