fix(gateway): 修复仅配置 provider retry 时 429 不重试

[Kayphoon]

Summary

• attempt.rs: 从 failover_rules/endpoint/provider 三级读取 max_retries 生成 attempt slots，支持配置 provider retry 后 429 自动重试，限制最大 99 并兼容 legacy 默认值 2
• request_candidate.rs: 封装 candidate extra_data 构造参数，修复 workspace clippy 的 too_many_arguments 检查

Verification

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test -p aether-scheduler-core
• cargo test --manifest-path apps/aether-gateway/Cargo.toml build_local_attempt_identities -- --nocapture
• cargo test --manifest-path apps/aether-gateway/Cargo.toml

[Kayphoon]

Superseded by scope-clean PR #338. This one included an unrelated request_candidate.rs clippy cleanup, so closing to keep the retry fix focused.