test: cover sweep-fee floor boundaries and add below-floor metric

[piotr-roslaniec]

Summary

Follow-up to #4194, addressing the confirmed, actionable findings from a multi-agent review of that PR. Stacked on feat/follower-sweep-fee-soft-check; rebase onto main once #4194 merges.

Scope is deliberately narrow: test coverage for the untested soft check and fee-floor boundaries, plus one small observability add. No behavior of the leader-side floor changes.

Changes

Follower soft check: tests + metric (was untested — review's lead item)

• Extract the below-floor decision from ValidateDepositSweepProposal into a pure checkSweepFeeFloor helper, and unit-test it directly (just below / exactly at / above the safe minimum, plus missing and zero fee) instead of scraping logger output. The check previously had zero behavioral tests — only a constant-drift guard — so a flipped Cmp or a dropped branch would pass CI silently.
• Emit a deposit_sweep_fee_below_floor_total counter on the follower path when a proposal is below the floor. The check was log-only; a metric lets operators alert on underpriced proposals during a mixed-version rollout instead of grepping node logs. Reuses the existing follower metricsRecorder; ValidateDepositSweepProposal's signature is unchanged and the node still signs the proposal.

Redemption fee: cap boundary tests

• EstimateRedemptionFee gains coverage for two boundary outcomes of the safe-minimum floor:
  • buffered fee above the Bridge max is bounded down to the cap (not an error);
  • a raw estimate already above the cap errors before the floor is applied.
• The second is intended behavior (documented in redemptions.go), but it trades liveness (redemption not attempted this round) for not broadcasting a fee the Bridge would reject. The test pins that tradeoff so it is a visible, regression-safe decision.

Reviewed but intentionally deferred (not fixed here)

• Follower compares against the static 5*vsize floor, not the buffered estimate. Catching "above floor but underpaid" proposals would require a live fee-oracle query inside proposal validation — the wrong layer, and it introduces non-determinism into validation. Hard enforcement belongs on-chain in the WalletProposalValidator.
• Soft check covers only 1 of the 4 floored flows (deposit sweeps, not redemptions / moving funds / moved-funds sweeps). Kept as-is per the decision to keep-and-improve the deposit-sweep check; flagged here so the asymmetry is a visible decision rather than an oversight.
• P2SH deposit vsize underestimation (both leader and follower use isWitness=true) — already documented as a code caveat in fee.go.
• Integer-truncation before the 25% buffer in applyWalletTxFeeFloor — a no-op for current callers (EstimateFee returns exact multiples); safe to defer.
• Nil-SweepTxFee branch — defensive; a nil fee fails at ABI packing before the soft check runs, so it is not reachable in production.
• Mirror constants (tbtc <-> tbtcpg) — already guarded by the drift test added in Add follower-side soft check for below-floor sweep fees #4194.

An overflow concern raised during review was rejected as logically unreachable: the int64(maxTotalFee) cast runs only inside uint64(totalFee) > maxTotalFee, which for an int64 totalFee implies maxTotalFee < MaxInt64, so the cast cannot wrap.

Testing

• go test ./pkg/tbtcpg/ — pass
• go test ./pkg/tbtc/ -run 'TestDepositSweepAction|TestCheckSweepFeeFloor|TestSweepFeeConstantsMirrorTbtcpg' — pass
• go vet / gofmt clean on both packages

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