json: reject null elements in closed enum containers

[fallintoplace]

What

• Reject null entries in closed enum lists and maps.

Why

• {"tiers":[null]} was accepted as the default enum value.
• ProtoJSON rejects null elements.

Implementation

• Return a malformed-value error from the shared decoder.
• Keep whole-field null handling.

[github-actions]

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[iainmcgin]

[claude code] Thanks for the fix and for testing it through proto2, editions and the no_std path. #691 edited the next line of the same function and has merged, so this branch needed a merge. Maintainer commits on top of yours: merges of main, with the one conflict resolved by keeping your null arm and the unknown-name wording from #691, and a changelog commit that puts the fragment in the house format with the PR number and adds a sentence saying that open-enum lists and maps still read a null element as 0. That open-enum half is tracked as a follow-up and is not part of this PR. Your code and tests are unchanged apart from the conflict resolution.