Add transcript parser tests

[edwin-zvs]

Summary

• factor Codex, Claude, and Antigravity native transcript parsing into event-returning helpers
• add fixture-style parser tests for message, tool-use, tool-result, cost, and skip cases
• keep runtime emit behavior unchanged by forwarding the returned semantic events

Testing

• rustfmt --edition 2024 --check crates/adapter-codex/src/main.rs crates/adapter-claude/src/main.rs crates/adapter-antigravity/src/main.rs
• git diff --check
• cargo test -p agentd-adapter-codex -p agentd-adapter-claude -p agentd-adapter-antigravity
• cargo check
• cargo build