Fix VerifyEvents to handle amounts exceeding int64 range

[karthikiyer56]

This PR fixes 2 issues:

1/
Replace int64 balance tracking with *big.Int in findBalanceDeltasFromEvents and all related functions. This fixes the unrecovered panic in MustParseInt64Raw when processing SAC events with i128 amounts that exceed int64 range (e.g., cumulative mints producing balances >2^63-1).

findBalanceDeltasFromEvents now returns an error instead of panicking on unparseable amounts. No exported API signatures changed.

Fixes #5929

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2/
Fix NewMaxFee() which was truncating the int64 fee-bump fee to uint32, silently corrupting any fee-bump fee above ~429.5 XLM.
Return type changed from uint32 to int64 (breaking change)

Fixes #5931

[Copilot]

Pull request overview

This PR updates the token transfer verification/reconciliation logic to correctly handle SAC event amounts that exceed the int64 range by switching internal balance-delta tracking from int64 to *big.Int, and by returning errors instead of panicking on invalid amounts. This addresses the VerifyEvents failure described in issue #5929.

Changes:

• Replace int64 delta maps with map[balanceKey]*big.Int across event/change delta aggregation and reconciliation paths.
• Add safe parsing of event amount strings into *big.Int, returning errors for invalid amounts instead of panicking.
• Add unit tests covering large (i128-range) amounts, invalid amounts, and updateBalanceMap behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
processors/token_transfer/verify_events.go	Switch delta tracking to *big.Int, add amount parsing with error returns, and compare maps using a big.Int comparer.
processors/token_transfer/verify_events_test.go	New tests for large SAC amounts, invalid amounts, fee/clawback, and map update behavior.
processors/token_transfer/token_transfer_processor.go	Update reconciliation logic to consume *big.Int delta maps and emit mint/burn events using big-int string amounts.

Comments suppressed due to low confidence (1)

processors/token_transfer/verify_events.go:167

• findBalanceDeltasFromEvents calls event.GetAsset() before switching on the concrete event type. TokenTransferEvent.GetAsset() panics on unknown event types, so this can still crash VerifyEvents even though the switch default now returns an error. To avoid unrecovered panics, restructure to avoid calling GetAsset() until after the event type is known (or use type-specific getters to access the asset).

	for _, event := range events {
		if event.GetAsset() == nil { // needed check for custom token events which won't have an asset
			continue
		}

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