fix: Read file-like KVS values before upload and reject unencodable ones #968

Job	Run time
PR title check	5s
Checks / Docstrings check	17s
Checks / Package check	19s
Checks / Actions lint check	29s
Checks / Markdown lint check	40s
Checks / Website lint check	33s
Checks / Spell check	6s
Checks / Image lint check	6s
Checks / Lint check / Lint check (3.11)	8s
Checks / Integration tests (3.14, ubuntu-latest)	2m 42s
Checks / Integration tests (3.11, ubuntu-latest)	2m 42s
Checks / Lint check / Lint check (3.14)	13s
Checks / Lint check / Lint check (3.13)	12s
Checks / Lint check / Lint check (3.12)	13s
Checks / Doc check / Docs check	1m 19s
Checks / Type check / Type check (3.14)	16s
Checks / Type check / Type check (3.12)	16s
Checks / Type check / Type check (3.13)	9s
Checks / Type check / Type check (3.11)	14s
Checks / Unit tests / Unit tests (windows-latest, 3.14)	1m 2s
Checks / Unit tests / Unit tests (ubuntu-latest, 3.12)	51s
Checks / Unit tests / Unit tests (ubuntu-latest, 3.14)	52s
Checks / Unit tests / Unit tests (windows-latest, 3.13)	1m 7s
Checks / Unit tests / Unit tests (windows-latest, 3.12)	1m 3s
Checks / Unit tests / Unit tests (windows-latest, 3.11)	1m 7s
Checks / Unit tests / Unit tests (ubuntu-latest, 3.13)	52s
Checks / Unit tests / Unit tests (ubuntu-latest, 3.11)	48s
	18m 41s

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