feat(vault): optimistic minting exposure cap and per-minter rate limits

solidity/contracts/test/TBTCVaultThrottleExemptStub.sol

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+// SPDX-License-Identifier: GPL-3.0-only
+
+pragma solidity 0.8.17;
+
+import "../vault/TBTCVault.sol";
+import "../bank/Bank.sol";
+import "../token/TBTC.sol";
+import "../bridge/Bridge.sol";
+
+/// @dev Test stub exercising the optimistic minting throttle exemption seam
+///      of `TBTCOptimisticMinting`. Allows marking arbitrary requesters as
+///      exempt from the optimistic minting rate limits.
+contract TBTCVaultThrottleExemptStub is TBTCVault {
+    mapping(address => bool) public throttleExempt;
+
+    constructor(
+        Bank _bank,
+        TBTC _tbtcToken,
+        Bridge _bridge
+    ) TBTCVault(_bank, _tbtcToken, _bridge) {}
+
+    function setThrottleExempt(address requester, bool exempt) external {
+        throttleExempt[requester] = exempt;
+    }
+
+    function _isOptimisticMintingThrottleExempt(address requester)
+        internal
+        view
+        override
+        returns (bool)
+    {
+        return throttleExempt[requester];
+    }
+}