feat: add complete audit log download helper

[yummybomb]

Summary

• add AuditLogs.Download for complete exports while retaining ExportChunk for low-level access
• use gzip-compressed JSON Lines for high-level downloads; raw format selection remains on ExportChunk
• verify checksums and reject missing, stalled, or cyclic cursor chains before writing each chunk
• preserve standard SDK HTTP retry behavior and make transfer retries configurable
• report download totals and optional progress, with partial-output behavior documented

Testing

• go test ./...
• go vet ./...

---

Note

Low Risk
New client-side SDK helper with no changes to existing APIs; risk is limited to callers adopting Download and handling documented partial-output behavior on failure.

Overview
Adds AuditLogs.Download, a high-level helper that streams a full gzip JSON Lines audit export to an io.Writer by repeatedly calling the existing ExportChunk API with cursor pagination.

Each chunk is validated before write: X-Content-Sha256 must match the body, pagination headers (X-Has-More, X-Row-Count, X-Next-Cursor) are strictly checked (including rejecting stalled or cyclic cursors), and invalid pagination fails before bytes are written. Transient read/checksum failures retry with capped exponential backoff via WithAuditLogDownloadMaxTransferRetries, while HTTP retries stay on the standard SDK request options (WithAuditLogDownloadRequestOptions). Optional WithAuditLogDownloadProgress reports cumulative bytes/chunks/rows after each verified chunk; docs note that errors can leave partial output on dst.

Tests cover multi-chunk success, checksum retry, cursor/header rejection, cursor cycles, HTTP vs transfer retry limits, and write failures.

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