Fix spurious printing of forecast objects during := by leveraging data.table 1.17.0 fix

[nikosbosse]

CLAUDE:

Summary

Issue #935 reported that forecast objects print their entire contents when columns are modified via :=. This is still reproducible on the current main branch despite workarounds added in #884.

Root cause

This is caused by a known data.table bug (Rdatatable/data.table#3029) where := triggers autoprinting for S3 subclasses of data.table that define custom print methods. The bug was fixed in data.table 1.17.0 via PR #6631.

However, even with data.table >= 1.17.0, the issue persists in scoringutils because of two interacting problems:

1. [.forecast wraps [.data.table and breaks autoprint suppression

The custom [.forecast method (added in #884) wraps [.data.table via NextMethod() and adds validation logic. This wrapping disrupts data.table's internal autoprint suppression mechanism. The method includes a self-described "hack" that skips validation for objects with ≤30 rows to work around print.data.table's internal subsetting, but this doesn't prevent the autoprint itself.

2. print.forecast outputs before calling NextMethod()

print.forecast prints a header (forecast type, forecast unit) before calling NextMethod() to dispatch to print.data.table. The data.table 1.17.0 fix correctly suppresses print.data.table's output during := autoprint, but print.forecast's header output has already been emitted by that point.

Reproducer

library(scoringutils)

ex <- data.table::copy(example_quantile)
cat("BEFORE\n")
ex[, model := paste(model, "a")]
cat("AFTER\n")
# Entire forecast object is printed between BEFORE and AFTER

Proposed fix

1. Bump data.table dependency from >= 1.16.0 to >= 1.17.0 in DESCRIPTION
2. Use shouldPrint() in print.forecast: Call data.table's internal shouldPrint(x) at the top of print.forecast() and return invisible(x) early if it returns FALSE. This ensures the forecast header is also suppressed during := autoprint, just as print.data.table suppresses the table content.
3. Remove the [.forecast hack: The 30-row threshold workaround in [.forecast was added specifically to avoid triggering validation during print.data.table's internal subsetting. With shouldPrint() handling the autoprint suppression, this hack is no longer needed.
4. Keep [<-.forecast, $<-.forecast, [[<-.forecast: These assignment methods serve a separate purpose (validation on modification) and are unrelated to the printing issue.

Verification

Tested locally with data.table 1.18.2.1. After applying the proposed fix:

• := operations on forecast objects produce no output (correct)
• Explicit print() calls still display the full forecast header + table (correct)

Relates to #935.