Guard Sync Transport (src/guard_sync_transport.rs)

Handles Guard-Cloud sync transport including DPoP proof signing, OAuth config resolution, endpoint validation, and bounded retry request logic over pinned TLS.

// Source: https://github.com/hashgraph-online/hol-guard | License: Apache-2.0 | Copyright You must retain, in the Source form of any Derivative Works
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//! Guard-Cloud sync transport: DPoP proof signing, request construction,
//! endpoint validation, and the bounded retry state machine.
//!
//! Mirrors `runner.py` `_validate_guard_sync_url` (:3994), `_guard_sync_headers`
//! (:4783), `_guard_sync_request` (:3810), `_guard_sync_request_with_nonce`
//! (:4823), `_sign_guard_dpop_proof` (:3701), `_dpop_nonce_from_http_error`
//! (:3785), `_is_timeout_error` (:4997), `_urlopen_with_sync_retries` (:5009),
//! and `oauth_client.py` `validate_guard_sync_endpoint` (:174) +
//! `resolve_guard_oauth_client_config` (:171) + `is_guard_oau

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