fix: Read file-like KVS values before upload and reject unencodable ones #969

Job	Run time
PR title check	11s
Checks / Spell check	7s
Checks / Markdown lint check	48s
Checks / Package check	19s
Checks / Actions lint check	32s
Checks / Website lint check	36s
Checks / Image lint check	6s
Checks / Integration tests (3.11, ubuntu-latest)	2m 55s
Checks / Docstrings check	14s
Checks / Doc check / Docs check	1m 29s
Checks / Lint check / Lint check (3.11)	8s
Checks / Lint check / Lint check (3.14)	13s
Checks / Lint check / Lint check (3.13)	17s
Checks / Type check / Type check (3.11)	14s
Checks / Type check / Type check (3.14)	12s
Checks / Type check / Type check (3.12)	14s
Checks / Type check / Type check (3.13)	16s
Checks / Unit tests / Unit tests (windows-latest, 3.11)	1m 6s
Checks / Unit tests / Unit tests (ubuntu-latest, 3.12)	49s
Checks / Integration tests (3.14, ubuntu-latest)	2m 59s
Checks / Unit tests / Unit tests (windows-latest, 3.13)	1m 11s
Checks / Unit tests / Unit tests (ubuntu-latest, 3.14)	49s
Checks / Unit tests / Unit tests (ubuntu-latest, 3.11)	49s
Checks / Lint check / Lint check (3.12)	8s
Checks / Unit tests / Unit tests (windows-latest, 3.12)	1m 6s
Checks / Unit tests / Unit tests (windows-latest, 3.14)	1m 13s
Checks / Unit tests / Unit tests (ubuntu-latest, 3.13)	46s
	19m 47s

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