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This fixes another regression in the previous commit that fixed other
known bugs.

Changes in #8002 introduced fixes for bugs with mangling JSON Schemas.
It also fixed a bug where the server would silently fail when clients
requested invalid formats. It also, unfortunately, introduced a bug
where the server would reject requests with an empty format, which
should be allowed.

The change in #8127 updated the code to allow the empty format, but also
reintroduced the regression where the server would silently fail when
the format was set, but invalid.

This commit fixes both regressions. The server does not reject the empty
format, but it does reject invalid formats. It also adds tests to help
us catch regressions in the future.

Also, the updated code provides a more detailed error message when a
client sends a non-empty, but invalid format, echoing the invalid format
in the response.

This commits also takes the opportunity to remove superfluous linter
checks.