feat(rust-provider): add HTTP retry for flagLog writes

[nicklasl]

Summary

• Add retry with exponential backoff to LogSender::send() in the Rust provider, matching the JS provider's flagLog retry behavior
• 3 attempts, 500ms base delay, 2x backoff, ±10% jitter
• Retries on 5xx, 408, 429, and network errors; no retry on other 4xx
• Respects Retry-After header from server (takes precedence over computed backoff)
• Warns (not errors) after retries exhausted since log failures shouldn't disrupt flag evaluation

Test plan

• Unit tests for retry on 503, 429, 408
• Unit tests for no-retry on 400, 403
• Unit test for Retry-After header respect
• Unit tests for jitter bounds and parse_retry_after
• is_retryable_status coverage for all relevant codes
• Full crate test suite passes (95 tests)
• clippy clean

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