Preserve ContentBlock metadata during JSON serialization

[abietic]

Summary

• preserve _meta and annotations when the generated ContentBlock.MarshalJSON shapes each wire variant
• update the generator so the fix survives schema regeneration
• cover text, image, audio, resource-link, and embedded-resource variants

Without this, unmarshalling a valid content block and marshaling it again silently drops standard annotation data and reserved extension metadata.

Verification

• GOTOOLCHAIN=go1.21.13 go test ./...
• GOTOOLCHAIN=go1.21.13 go test -race ./...
• GOTOOLCHAIN=go1.21.13 go -C cmd/generate test ./...
• regenerated twice and confirmed an identical diff
• git diff --check

[abietic]

@ThomasK33 When you have a chance, could you review this wire-compatibility fix? I have now verified the root and generator modules, including -race, with the repository minimum Go 1.21.13 toolchain.