feat: request retries

[kewynakshlley]

Summary

• RequestOptions: new maxRetries(int). It rejects negatives, and when unset the request inherits the client's default.
• HttpClient: new constructor HttpClient(OkHttpClient, String, int maxRetries) and a getMaxRetries() getter. execute now runs an attempt loop, and the per-request timeout applies to each attempt. The
  two-argument constructor still means no retries.
• Resend.Builder: new maxRetries(int). Like the timeouts, it can't be combined with httpClient(...), and the error message and javadocs say so.
• README: a new "Retries and timeouts" section. It also documents the per-request timeout from PR 1, which wasn't in the README yet.

[cubic-dev-ai]

All reported issues were addressed across 8 files

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[cubic-dev-ai]

0 issues found across 5 files (changes from recent commits).

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

<sub>Requires human review: Adds opt-in request retries and per-request timeouts to the HTTP client, plus builder/API plumbing and docs. Needs human sign-off on the dismissed blocking-wait tradeoff: retries block the calling thread and Retry-After is capped at 30s per attempt.

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