fix(streamable-http): return spec-mandated 405 for pre-session GET (Closes #3102) #3492

Job	Run time
checks / pre-commit	49s
checks / readme-snippets	13s
checks / docs	28s
checks / test (3.11, lowest-direct, ubuntu-latest)	1m 11s
checks / test (3.10, locked, ubuntu-latest)	1m 17s
checks / test (3.14, locked, windows-latest)	1m 50s
checks / test (3.14, lowest-direct, windows-latest)	1m 45s
checks / test (3.10, lowest-direct, ubuntu-latest)	1m 15s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 22s
checks / test (3.10, locked, windows-latest)	2m 19s
checks / test (3.12, locked, ubuntu-latest)	1m 28s
checks / test (3.12, locked, windows-latest)	2m 1s
checks / test (3.11, locked, ubuntu-latest)	1m 16s
checks / test (3.10, lowest-direct, windows-latest)	2m 0s
checks / test (3.14, locked, ubuntu-latest)	1m 13s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 8s
checks / test (3.11, lowest-direct, windows-latest)	1m 58s
checks / test (3.13, locked, ubuntu-latest)	1m 15s
checks / test (3.13, locked, windows-latest)	1m 59s
checks / test (3.12, lowest-direct, windows-latest)	1m 58s
checks / test (3.13, lowest-direct, ubuntu-latest)	1m 10s
checks / test (3.11, locked, windows-latest)	2m 28s
checks / test (3.13, lowest-direct, windows-latest)	2m 4s
all-green	4s
	34m 31s

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