Oversized "Delete" warning listing every object when deleting a group whose results live in another group #451

Symptoms

Deleting a group of signals or images whose processing results were kept in another group shows an oversized "Delete" warning with an OK button only:

Some operations could not be reconnected after deletion:
• "fc1deab8-bde0-…" has dependent operations but no valid source to reconnect to — downstream results are left unchanged.
• … (one line per deleted object)

It appears even when the History panel is not recording. Typical scenario: a group of raw signals, processed as a group (results in a new group), then the raw group is deleted. With 5 signals, the message lists 5 lines of raw object UUIDs; with many objects, the dialog becomes huge. Present in DataLab 1.3.0.

Root causes

History object tracking is always enabled: after each deletion from the signal/image panels, reconnect_chain_after_removal tries to rewire the surviving results to the source of each deleted object.

1. plan_reconnection produced a warning for every deleted object with surviving dependents and no alive source, including objects that never had a source: raw data (created, loaded) or objects processed while history was not recording. Nothing could be reconnected there, yet the label fell back to the raw UUID.
2. show_reconnection_warnings listed one line per deleted object, without deduplication nor size limit (the same operation title was repeated for each object of a group).
3. find_reconnection_source took the first recorded selection of any producing action as its source, including UI creation/loading actions, which have no source.

Proposed fix

• Only processing (compute) actions have a source; UI producers report none.
• Warn only when a recorded processing step loses its source; raw/unrecorded objects leave their dependents unchanged silently.
• One message listing each operation once, capped at 10 lines ("... and N more").
• Regression tests (group deletion with and without recording), unit tests, documentation (EN/FR) and release notes.