feat: production-grade retry/backoff with error classification and partial output protection

[danielxxomg]

Summary

Replaces the naive retry logic in src/model.ts with a production-grade retry engine that handles transient network errors, server failures, and mid-stream disconnects gracefully.

Problems Solved

1. Thundering herd: Original uses exponential backoff (2s * 2^n) without jitter — all clients retry at the same time
2. No error classification: Retries on auth/quota errors (insufficient credit, unauthorized) that will never succeed
3. Duplicate content on reconnect: Mid-stream reconnects can emit text/tool-calls twice if content was already sent downstream
4. No HTTP status retry: 429 (rate limit) and 5xx (server error) are not retried

Changes

src/model.ts (327 → 532 lines)

• extractMessage() — handles Error, string, and plain SSE error objects for classification
• NON_RETRYABLE_PATTERNS (18 patterns) — auth/quota/validation errors, never retry
• RETRYABLE_PATTERNS (24 patterns) — network/server/transient errors
• isRetryableStatus() — retries HTTP 429 and 5xx
• backoffDelay() — fixed schedule [1s, 2.5s, 5s] with ±25% jitter (replaces exponential 2s * 2^n)
• fetchOnce() — single fetch for mid-stream reconnects (no double retry counting)
• emittedContent tracking — gates reconnect: if text/reasoning/tool-call already emitted, aborts reconnect to prevent DUPLICATE content
• shouldRetry() — centralized decision: checks aborted, emittedContent, attempt count, and retryable classification
• partialOutputError() — clear error message when reconnect would cause duplicates
• wrapAsError() — normalizes any error shape into a real Error object
• buildHttpError() — parses JSON error bodies properly

Tests

• 9 new + 2 updated tests in tests/unit/model.test.ts
• New mockFetchRetrySequence() helper in tests/helpers/mocks.ts
• 22 model tests passing, 88 total tests passing

Backoff Schedule Comparison

Attempt	Original (exponential)	New (fixed + jitter)
1	2s	~1s (0.75–1.25s)
2	4s	~2.5s (1.9–3.1s)
3	8s	~5s (3.75–6.25s)
4	16s	~5s (3.75–6.25s)

Test Results

88 pass
0 fail
189 expect() calls
Ran 88 tests across 6 files.