ci: fail on unsupported JSON initializer patterns

[agammann]

The two JSON initializer guards in test_format print a diagnostic when they find a prohibited pattern, but then run echo exit 1. Because that command succeeds, the checks do not fail the job when they detect a violation.

Replace echo exit 1 with exit 1 in both guards so a match stops the job as intended. The existing search patterns and diagnostics are preserved.

Validation used the exact guard commands in isolated Bash scripts with set -e. A fixture containing auto other = nlohmann::json({}); and an indented nlohmann::json fixture({}); made both original guards print their diagnostic and exit 0. Both corrected guards exit 1 for that fixture and exit 0 for an allowed auto allowed = j_empty; fixture. Both corrected guards also pass against the current repository source. git diff --check passed.

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