Both public assembly paths took the tier on trust: a bare "none" from
JSON or a hand-built SourceConfig stayed a str, and `is Effort.NONE`
then read it as a contradiction and crashed on `.value` while wording
the error -- the caller got an AttributeError where a ValueError was
promised, and on the request side that unclassified exception walked
straight through the transport's ThinkingUnsupportedError catch and the
retry classifier.
Normalize at the two entrances instead, matching what the .env path has
always done, and let EFFORT_FALLBACK be spelled with the same freedom as
its neighbour.
iomgaa
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