fix(runner): keep --output json parseable and contain vector-file crashes

[AlexBill01]

vector_runner: keep --output json machine-readable and survive malformed vector files

CI consumes this runner's stdout as a JSON artifact:

vector_runner.py --output json > results-<adapter>.json

Two failure modes can silently destroy that artifact today.

1. Stray print() calls bypass the json guard

VectorRunner.log() exists precisely to suppress human-readable output when
output_format == "json", but run_vector_file() and _record_result() still
call print() directly in a few places (missing vector file, vector file with
no commands, and the verbose per-test lines). Any of those lines lands in
front of the JSON document, so merge_vector_results.py fails to parse the
artifact and the adapter's entire pass/fail breakdown collapses into a single
"Could not read results" failure.

All of these call sites now go through self.log(), which is a no-op in json
mode and unchanged in text mode.

2. A malformed vector file aborts the whole run

run() invoked run_vector_file() unguarded. One vector file with invalid
JSON — or a scenario missing a required key like input or expected —
raised straight out of main(), so in json mode nothing was written to
stdout and every check that had already run was lost.

Per-adapter and per-operation errors were already contained
(run_adapter / run_operation_timed catch exceptions and record failures);
file-level errors are now contained the same way: the runner records a failing
vector_file_error check for that file and keeps going, and the overall run
correctly reports fail.

Verification

$ python3 scripts/test_vector_runner.py
Ran 9 tests ... OK            # 6 existing + 3 new
$ python3 scripts/vector_runner.py --adapter typescript --vector base64url --output json | python3 -m json.tool
# parses cleanly, status "pass", 30 checks

The new tests pin down: no stdout leakage for command-less vectors, no stdout
leakage for missing vector files, and a malformed vector file producing a
failing check (not a crash