Send mint-token requests with a proper Content-Type (#18334)
## Summary
Using `.body()` meant that the request was sent without a
`Content-Type`, whereas we want to explicitly communicate that the
mint-token request is a JSON payload. Using `.json()` causes request to
send the `Content-Type` header by default.
## Test Plan
Existing coverage.
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Signed-off-by: William Woodruff <william@astral.sh>
