reflect: preserve oneof members on decode errors

[fallintoplace]

What this does

DynamicMessage now decodes a singular oneof replacement into a temporary value before clearing sibling members. If decoding fails, the previously valid oneof member remains set. Existing singular message values are cloned before wire merging as well, so malformed repeated message occurrences do not mutate the stored value.

Coverage

• truncated replacement varints
• truncated nested-message payloads
• mismatched group terminators
• invalid UTF-8 in a replacement string

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• cargo check --workspace --all-features

[github-actions]

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