perf: discriminate JSONRPCMessage union by key presence instead of smart-union scoring #3547

Job	Run time
checks / docs	51s
checks / readme-snippets	16s
checks / test (3.10, lowest-direct, ubuntu-latest)	1m 33s
checks / pre-commit	55s
checks / test (3.12, locked, windows-latest)	1m 55s
checks / test (3.11, locked, windows-latest)	1m 50s
checks / test (3.12, locked, ubuntu-latest)	1m 41s
checks / test (3.14, lowest-direct, windows-latest)	1m 53s
checks / test (3.14, locked, ubuntu-latest)	1m 21s
checks / test (3.11, locked, ubuntu-latest)	1m 20s
checks / test (3.10, locked, ubuntu-latest)	1m 25s
checks / test (3.13, locked, windows-latest)	1m 56s
checks / test (3.13, lowest-direct, ubuntu-latest)	1m 23s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 22s
checks / test (3.14, locked, windows-latest)	1m 57s
checks / test (3.11, lowest-direct, windows-latest)	1m 52s
checks / test (3.13, locked, ubuntu-latest)	1m 14s
checks / test (3.13, lowest-direct, windows-latest)	1m 59s
checks / test (3.11, lowest-direct, ubuntu-latest)	1m 21s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 9s
checks / test (3.12, lowest-direct, windows-latest)	2m 17s
checks / test (3.10, locked, windows-latest)	1m 44s
checks / test (3.10, lowest-direct, windows-latest)	1m 38s
all-green	5s
	34m 57s

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