feat: authenticate calls to transaction api

[klejeune]

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Note

Medium Risk
Adds optional Authorization headers to all Smart Transactions network calls when a token provider is configured, which can affect request authentication and backend behavior if misconfigured. Risk is limited to the STX fetch wrapper and is covered by new header-matching tests.

Overview
Adds optional request authentication for Smart Transactions. SmartTransactionsController now accepts an optional getBearerToken callback and, when provided, attaches Authorization: Bearer <token> to requests targeting the Transaction API (API_BASE_URL) and Sentinel endpoints (SENTINEL_API_BASE_URL_MAP).

Tests are extended to assert the Authorization header is sent for both cancelSmartTransaction and fetchLiveness when getBearerToken is configured.

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