feat(failover): reactive failover walker

[quangdang46]

Summary

Implements the reactive failover walker for A8 — a per-session state machine that orchestrates automatic provider failover when errors occur mid-stream.

Architecture

Composes three existing pieces:

• classify_failover_error_message_structured (failover.rs) — classifies errors into FailoverDecision + ErrorCode
• pick_next_fallback_route (fallback_pick.rs) — picks next best route with 3-tier ranking
• New ReactiveFailoverWalker (failover_walker.rs) — adds the orchestration layer: per-session tracking, cooldowns, equivalence checks, internally-aborted bookkeeping

New File

crates/jcode-provider-core/src/failover_walker.rs — 536 lines, 11 tests

Key types

• WalkState — per-session state (original_model, current_model, fallback_index, failed_models with Instant cooldowns, attempt_count)
• PreparedFallback — result of fallback preparation
• WalkResult — full walk result (should_failover, new_model, decision, error_code, message)
• ReactiveFailoverWalker — main struct

Main methods

• walk_failover() — classify error → decide → pick fallback → update state → return result
• prepare_fallback() — pick next candidate with max-attempts check
• find_next_available_fallback() — walk chain respecting cooldowns + equivalence
• record_failure/record_success — cooldown management
• is_internally_aborted/mark_internally_aborted — for consumer integration

Test coverage

1. Session lifecycle (register → get_state → unregister)
2. Cooldown tracking (failure → cooldown → success clears)
3. Fallback skips equivalent models
4. Fallback skips cooldown models
5. Max attempts reached
6. Rate-limited failover (picks fallback)
7. Context-length retries (RetryNextProvider, different model)
8. Internally-aborted tracking
9. No routes available
10. models_equivalent helper
11. Unregister cleans internally_aborted set

Plan

docs/pr-plans/A8-failover-walker.md

Tracking

• PARITY.md: entries 4.3 (Provider failover) and 9.8 (Reactive failover walker) updated
• PR_BACKLOG.md: A8 marked ✅ Done