UN-3235 [FEAT] Add retry with exponential backoff for transient failures

[muhammad-ali-e]

What

• Add automatic retry with configurable exponential backoff to both V1 and V2 clients
• Add comprehensive retry unit tests and improve integration test robustness

Why

• Transient server errors (5xx) and rate limits (429) cause unnecessary failures
• Network interruptions (ECONNRESET, ETIMEDOUT) should be retried transparently
• Highlight integration test was timing out at 20s; webhook test was hardcoded to a specific URL

How

• Integrated axios-retry into both client constructors with configurable parameters (maxRetries, initialDelay, maxDelay, backoffFactor, jitter)
• Each client creates its own axios.create() instance to isolate retry config
• Synchronous whisper (timeout > 0) and registerWebhook disable retries to avoid duplicate processing
• File streams are re-created via onRetry callback to handle consumed streams
• Added 577-line test/retry.test.js with mocked adapter tests
• Fixed integration test timeouts and conditional test skipping

Can this PR break any existing features. If yes, please list possible items. If no, please explain why. (PS: Admins do not merge the PR without this section filled)

• No breaking changes. The retry behaviour is additive with sensible defaults (4 retries). Existing callers that never hit 5xx/429/network errors see zero difference. Retries can be disabled by passing maxRetries: 0.

Database Migrations

• None

Env Config

• Added optional LLMWHISPERER_WEBHOOK_URL in test/sample.env (test-only, not required for production)

Relevant Docs

• axios-retry: https://github.com/softonic/axios-retry

Related Issues or PRs

• https://zipstack.atlassian.net/browse/UN-3235

Dependencies Versions

• Added axios-retry@^4.5.0

Notes on Testing

• npm test passes with mocked retry tests (no API key needed)
• Integration tests skip gracefully when LLMWHISPERER_API_KEY is unset
• Webhook test skips when LLMWHISPERER_WEBHOOK_URL is unset

Screenshots

N/A

Checklist

I have read and understood the Contribution Guidelines.