feat(validators): clarify error when content-length is missing

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Fixes #2020.

Changes proposed in this pull request:

• Replace the "RequestBody is required" 400 message with "RequestBody is empty or the Content-Length header is missing or zero", so callers can tell their client omitted Content-Length rather than actually sending an empty body.

[SAY-5]

@pre-commit-ci run

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Note: the pre-commit.ci failure is a pre-existing upstream issue - the same hook also fails on the spec-first/connexion main branch itself (not related to this change). All hooks pass locally.

[SAY-5]

@pre-commit-ci run