test: validate JSON in RPC help examples #36003

Resolves bitcoin/bitcoin#35864

Summary

HelpExampleRpc builds JSON-RPC curl examples from literal argument strings. This change adds a functional-test guard that parses every generated curl payload as JSON, preventing future malformed examples from being published in RPC help text.

The guard exposed and this PR fixes malformed examples across blockchain, networking, mempool, wallet, and wallet-backup RPC help. The fixes cover missing commas, bare string placeholders, a JSON address-array example, and Windows path escaping.

Testing

• python3 -m py_compile test/functional/rpc_help.py
• rpc_help.py functional test: passed
• ctest --test-dir build --output-on-failure --parallel 2: 100% passed, 0 failed out of 372 tests; 1 test skipped (script_assets_tests)
• Bitcoin Core build with wallet, IPC, ZMQ, and tests enabled: passed

The regtest RPC smoke workflow was also run separately with local-only disposable state; it mined 101 blocks, validated a signed transaction with testmempoolaccept, confirmed it, and shut the daemon down automatically.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36003.

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