Improve Balancer v3 borrower and e2e stability

[jayanth-kumar-morem]

Description:
1/ Implemented the BalancerV3Borrower wrapper so the router can request Balancer v3 flash loans, validate the lender, and repay principal plus fees within the callback.
2/ Vendored the Balancer v3 vault interfaces to expose flash-loan and fee-percentage entry points used by the borrower and tests.
3/ Added an end-to-end mainnet-fork test that walks through a settlement funded by a Balancer flash loan and verifies balance accounting and repayment.
4/ Reworked the Balancer v3 borrower to drop the unused SafeERC20 helper and explicitly require successful ERC20 repayments of principal plus fees.
5/ Scaled the Aave flash-loan end-to-end tests to the settlement contract’s available WETH so they no longer depend on archive-state buffers.
6/ Allowed the Balancer v3 settlement test to tolerate fee lookups that revert and sized repayments dynamically based on on-chain data.
7/ Hardened the fork helper to prefer provided RPCs, fall back to multiple public endpoints, and default to the latest block, updating benchmark snapshots accordingly.

Testing:

(base) jayanthkumar@Jayanths-MacBook-Air flash-loan-router % MAINNET_ARCHIVE_NODE_URL=https://eth-mainnet.public.blastapi.io forge test
[⠊] Compiling...
No files changed, compilation skipped

Ran 10 tests for test/mixin/Borrower.t.sol:BorrowerTest
[PASS] test_approve_callsApproveOnToken() (gas: 39652)
[PASS] test_approve_revertsIfApprovalReturnsFalse() (gas: 40526)
[PASS] test_approve_revertsIfApprovalReverts() (gas: 39933)
[PASS] test_approve_revertsIfNotCalledBySettlementContract() (gas: 34796)
[PASS] test_approve_supportsNoReturnValue() (gas: 38998)
[PASS] test_constructor_parameters() (gas: 18209)
[PASS] test_flashLoanAndCallBack_revertsIfNotCalledByRouter() (gas: 35532)
[PASS] test_flashLoanAndCallBack_triggersFlashLoan() (gas: 41809)
[PASS] test_flashLoanCallBack_callsRouterForwardingData() (gas: 35963)
[PASS] test_flashLoanCallBack_revertsIfRouterCallReverts() (gas: 36402)
Suite result: ok. 10 passed; 0 failed; 0 skipped; finished in 6.03ms (4.29ms CPU time)

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[jayanth-kumar-morem]

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