replace production panics with graceful error handling in common crate

[adilhafeez]

Summary

• open_ai.rs: Replace panic!("Unsupported content type") in Display impl with silently skipping unknown content types (same as ImageUrl handling) — prevents crash when logging messages with new content types
• ratelimit.rs: Replace panic!("repeated selector") with log::error! and skip the duplicate — prevents crash on config typo with duplicate rate limit selectors
• http.rs: Replace panic!("Duplicate http call") with log::warn! — prevents crash if proxy-wasm runtime generates a duplicate call token ID

Also removes unused core::{panic, str} import that became dead code after the fix.

Test plan

• cargo test --lib -p common — all 36 tests pass
• cargo clippy --locked --all-targets --all-features -- -D warnings — clean
• cargo fmt --all -- --check — clean