perf: discriminate JSONRPCMessage union by key presence instead of smart-union scoring #3830

Job	Run time
checks / readme-snippets	15s
checks / docs	55s
checks / pre-commit	47s
checks / test (3.10, lowest-direct, ubuntu-latest)	1m 30s
checks / test (3.11, lowest-direct, windows-latest)	1m 58s
checks / test (3.11, locked, ubuntu-latest)	1m 28s
checks / test (3.11, lowest-direct, ubuntu-latest)	1m 10s
checks / test (3.10, locked, windows-latest)	2m 2s
checks / test (3.10, locked, ubuntu-latest)	1m 30s
checks / test (3.10, lowest-direct, windows-latest)	1m 57s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 16s
checks / test (3.12, locked, windows-latest)	2m 6s
checks / test (3.14, locked, ubuntu-latest)	1m 23s
checks / test (3.14, lowest-direct, windows-latest)	2m 3s
checks / test (3.13, lowest-direct, ubuntu-latest)	1m 23s
checks / test (3.13, locked, windows-latest)	2m 19s
checks / test (3.13, locked, ubuntu-latest)	1m 26s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 27s
checks / test (3.12, lowest-direct, windows-latest)	1m 36s
checks / test (3.14, locked, windows-latest)	1m 57s
checks / test (3.11, locked, windows-latest)	1m 55s
checks / test (3.12, locked, ubuntu-latest)	1m 44s
checks / test (3.13, lowest-direct, windows-latest)	2m 32s
all-green	5s
	36m 44s

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