feat(STX-501): add remote feature flag routing to getAPIRequestURL for sentinel migration

[rarquevaux]

Summary

• Adds remote feature flag–controlled routing to getAPIRequestURL() via an optional useSentinel parameter
• Gates each of the four sentinel endpoints (BATCH_STATUS, GET_FEES, SUBMIT_TRANSACTIONS, CANCEL) behind a dedicated LaunchDarkly flag
• All flags default to false — no production traffic shifts on merge; each endpoint can be switched independently

Test plan

• yarn test — all tests pass
• yarn lint:fix — 0 errors

---

Note

Medium Risk
Touches Smart Transactions fee/status/submit/cancel API routing; while gated by remote flags defaulting off, misconfigured flags could shift traffic to new Sentinel endpoints and affect transaction flows.

Overview
Adds remote-feature-flag controlled routing to send Smart Transactions GET_FEES, SUBMIT_TRANSACTIONS, CANCEL, and BATCH_STATUS requests to Sentinel /v1/... endpoints on supported chains.

SmartTransactionsController now reads per-endpoint migration flags (e.g., stxMigrationGetFees) and passes a new useSentinel option into getAPIRequestURL, which falls back to the legacy API_BASE_URL when disabled or unsupported. Unit tests expand coverage for the new useSentinel routing and fallback behavior.

^{Reviewed by Cursor Bugbot for commit 43e8d91. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, have a team admin enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 0c8d6bd. Configure here.}