chore: unify test naming convention across codebase

[chaodu-agent]

Context

Current test naming is inconsistent — some use test_ prefix, some don't:

// With prefix
fn test_generate_config_contains_sections()
fn test_validate_bot_token_ok()

// Without prefix
fn estimate_tokens_empty()
fn pack_arrival_event_single()

Proposal

Standardize on no test_ prefix (Rust convention — #[test] + mod tests already provides context) with descriptive <subject>_<scenario>_<expected> format.

Example: estimate_tokens_empty_returns_zero()

Scope

All #[test] functions in src/.

Raised during PR #951 review.

[github-actions]

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