[agent] Mark enums non-exhaustive and return iterators

[tlepoint]

Summary

• annotate public enums with #[non_exhaustive] for future-proofing
• avoid Vec allocations by returning iterators for default parameters and context chains
• adjust tests and benchmarks to new iterator APIs

Testing

• cargo +nightly fmt --all
• cargo clippy --all-targets -- -D warnings
• cargo test

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https://chatgpt.com/codex/tasks/task_e_68ab8465018c8323a282477ce91603a2