descriptor: reject missing reserved range bounds

[fallintoplace]

What changed

• Reject message and enum reserved ranges with a missing start or end bound.
• Keep complete ranges and the existing range normalization behavior unchanged.
• Return the original optional bounds in PoolError::InvalidReservedRange.

Tests

• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all -- --check
• task lint-md

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}