descriptor: reject duplicate oneof names

[fallintoplace]

What changed

• Reject duplicate oneof names within a message in DescriptorPool.
• Add a precise PoolError variant and display message.
• Add hand-built descriptor tests for rejection, transactional behavior, and distinct names.

Checks

• cargo test -p buffa-descriptor --features reflect --test pool_e2e
• cargo test -p buffa-descriptor --all-features
• cargo clippy -p buffa-descriptor --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check

[github-actions]

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