test(resilience): characterize Brainarr circuit breaker behavior

[RicherTunes]

Summary

Add 26 characterization tests documenting current circuit breaker semantics for WS4.2 migration decision-making.

Test Coverage by Category

Keying Scheme (2 tests)

• Documents "ai:{provider}:{modelId}" format from BreakerRegistry

Failure Classification (4 tests)

• TaskCanceledException → failure ⚠️ CRITICAL: may trip during shutdown
• TimeoutException → failure
• HttpRequestException → failure
• 4xx client errors → excluded via string matching ⚠️ IMPORTANT: brittle detection

Dual-Trip Logic (2 tests)

• Consecutive failures threshold: ≥5 opens circuit
• Failure rate threshold: ≥50% with ≥10 minimum throughput

Half-Open Transitions (3 tests)

• 3 successes to close
• Any failure immediately re-opens
• Concurrency cap during half-open

Minimum Throughput Guard (1 test)

• Rate-based opening disabled below 10 operations

Windowing (2 tests)

• CircularBuffer sliding window behavior
• Statistics API

Configuration Constants (3 tests)

• Default, Aggressive, Lenient presets

Key Findings for WS4.2

1. CRITICAL: TaskCanceledException treated as failure - can trip breaker during host shutdown, request aborts, or client timeouts
2. IMPORTANT: 4xx detection via ex.Message.Contains("4") && ex.Message.Contains("Bad Request") is brittle and locale-dependent
3. IMPORTANT: Dual-trip logic (consecutive OR rate) + minimum throughput + half-open concurrency cap must map to Common breaker

WS4.2 Migration Path

These tests become the contract that WS4.2 must satisfy:

1. Adapter layer in Brainarr (no behavior change) + run characterization tests against it
2. If Common lacks semantics, extend Common then delete Brainarr implementation

Test Plan

• All 26 tests pass locally
• No sleeps/polling - uses immediate assertions
• Brittle behaviors documented in test comments

🤖 Generated with Claude Code