test(client): add API error scenario and rate limiting tests

[joelp172]

Summary

The test suite lacks coverage for API failure scenarios — rate limiting, timeouts, transient errors, and connection failures are not tested.

Current Behaviour

• Mock server only returns success responses
• No tests for 429, 500, 502, 503, timeout, or connection refused scenarios
• No tests for retry behaviour or backoff logic
• No tests for concurrent API calls

Proposed Test Cases

• 429 Rate Limit response triggers retry with backoff
• 500 Internal Server Error triggers retry
• 502/503 Gateway Error triggers retry
• 400 Bad Request is not retried
• 401 Unauthorized is not retried
• Connection timeout triggers retry
• Connection refused triggers retry
• Max retry limit is respected
• Retry-After header is parsed and respected
• Concurrent API calls don't interfere with each other
• Mock server can simulate intermittent failures

Acceptance Criteria

• Mock server supports configurable error injection
• All retryable error types tested
• Non-retryable errors verified to not retry
• Backoff timing validated (exponential increase)
• Tests pass reliably in CI

Phase

P2 — Testing Gaps (Phase 2: Trust)

Dependencies

• Depends on feat(client): add rate limit (429) handling with exponential backoff #47 (rate limit handling) and feat(controller): add exponential backoff for transient API errors #49 (exponential backoff)