fix: use `ASGIConnection` instead of `Request` for `flash` #3758

Job	Run time
mypy	1m 13s
pyright	56s
slotscheck	31s
validate	12s
Test server integration (uvicorn<0.27.0)	53s
Test server integration (uvicorn>=0.27.0)	53s
Test Minimal Application with Base Dependencies	34s
Test Minimal Pydantic 1 application	44s
test-platform-compat	0s
test (3.10) / test	2m 4s
test (3.11) / test	1m 52s
test (3.12) / test	3m 32s
test (3.8) / test	2m 51s
test (3.9) / test	1m 57s
build-docs	3m 0s
upload-test-coverage	15s
codeql	3m 16s
sonar	1m 6s
	25m 49s

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