Mock client: large values of `minimum` for random string generators

[mkows]

This is a follow-up of #515

minimum and maximum are represented as Long -- this is problematic when generating strings of length Long.MaxValue (for e.g. minimum = Long.MaxValue)

Should the upper bound of string value (some 1,000 - 10,000 for max string length) generated by the mock would be more appropriate.

As // TODOs in:
https://github.com/apicollective/apibuilder-generator/pull/515/files#diff-a9af02212db31af17f4fd4ba39f8ea60R53
https://github.com/apicollective/apibuilder-generator/pull/515/files#diff-a9af02212db31af17f4fd4ba39f8ea60R68
https://github.com/apicollective/apibuilder-generator/pull/515/files#diff-cc693e198c99b6062e9bebc81872e19dR31

Other TODOs can be safely removed:
https://github.com/apicollective/apibuilder-generator/pull/515/files#diff-cc693e198c99b6062e9bebc81872e19dR11
https://github.com/apicollective/apibuilder-generator/pull/515/files#diff-cc693e198c99b6062e9bebc81872e19dR14