test: run the PCAP-NG JSON report test now that keys are escaped (#1795)

[JarryShaw]

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Summary

tests/integration/test_pcapng_end_to_end.py skipped PcapngUnescapedKeyTests.test_json_report_of_a_decryption_secrets_block_parses as "blocked on dictdumper". #1158 (for #1152) made pcapkit escape JSON mapping keys itself, so the block was no longer real.

• Skip removed. The class and test docstrings now say what the test guards: the JSON report parses because pcapkit escapes keys, although dictdumper still writes them raw. Line-number citations are replaced by symbol names.
• HAS_RUNTIME guard added, matching the sibling classes, so a checkout without the runtime dependencies skips rather than fails. tests/integration/_helpers.py now says every subclass carries the guard.
• Bite: with escape_key's JSON branch turned into a no-op, the test fails with JSONDecodeError. With HAS_RUNTIME false, it skips.
• Runs: the file has 7 passed; tests/project and tests/test_tier_guard.py pass.

Closes #1795

[JarryShaw]

Verdict: GOOD TO GO at 58da7d377. A Sonnet cross-review and re-check (author: Opus). The skip removal is right: #1158 escapes the keys. The new HAS_RUNTIME guard matches its siblings, and an AST scan finds no unguarded subclass left. The test fails with escaping disabled and skips without the runtime dependencies. tests/project and the tier guard pass.