revert: feat: add binary codec with backward-compatible migration (#5419)

[gacevicljubisa]

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• I have read the coding guide.
• My change requires a documentation update, and I have done it.
• I have added tests to cover my changes.
• I have filled out the description and linked the related issues.

Description

Reverts #5419 (commit f53887f). Conflicts in accounting.go and migration.go from later commits (#5439, #5415, #5477) were resolved manually.

Open API Spec Version Changes (if applicable)

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• This PR contains code that has been generated by an LLM.
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[gacevicljubisa]

Reasons for reverts are bellow.

Here are issues that are detected in the logs of the public-testnet related to this:

"time"="2026-06-12 09:45:02.942100" "level"="warning" "logger"="node/accounting" "msg"="accounting: notifyrefreshmentsent failed to decrease originated balance" "error"="failed to load originated balance: failed decoding value Int.GobDecode: encoding version 24 not supported"
"time"="2026-06-12 09:45:02.945626" "level"="warning" "logger"="node/accounting" "msg"="increase balance; failed to decrease originated balance" "error"="failed to load originated balance: failed decoding value Int.GobDecode: encoding version 22 not supported"
"time"="2026-06-12 09:45:02.974804" "level"="warning" "logger"="node/accounting" "msg"="increase balance; failed to decrease originated balance" "error"="failed to load originated balance: failed decoding value Int.GobDecode: encoding version 24 not supported"

And if we run this tests, they will fail:

// TestUnmarshalBinaryLegacyFormats verifies that UnmarshalBinary accepts the
// two legacy on-disk encodings the statestore produced before the Gob codec:
// unquoted decimals from json.Marshal(*big.Int) (including negatives, which
// triggered "encoding version 22 not supported") and quoted decimal strings
// from json.Marshal(bigint.BigInt). This is the backward-compatibility path
// that lets nodes read balances written by older code regardless of migration.
func TestUnmarshalBinaryLegacyFormats(t *testing.T) {
    t.Parallel()

    tests := []struct {
        name string
        data []byte
        want *big.Int
    }{
        {"decimal positive", []byte("123456"), big.NewInt(123456)},
        {"decimal negative", []byte("-123456"), big.NewInt(-123456)},
        {"decimal zero", []byte("0"), big.NewInt(0)},
        {"quoted positive", []byte(`"123456"`), big.NewInt(123456)},
        {"quoted negative", []byte(`"-987654321"`), big.NewInt(-987654321)},
    }

    for _, tc := range tests {
        t.Run(tc.name, func(t *testing.T) {
            t.Parallel()

            var got bigint.BigInt
            if err := got.UnmarshalBinary(tc.data); err != nil {
                t.Fatalf("UnmarshalBinary(%q): %v", tc.data, err)
            }
            if got.Cmp(tc.want) != 0 {
                t.Fatalf("got %v, want %v", got.Int, tc.want)
            }
        })
    }
}

func TestUnmarshalBinaryInvalidDecimal(t *testing.T) {
    t.Parallel()

    var w bigint.BigInt
    if err := w.UnmarshalBinary([]byte("not-a-number")); err == nil {
        t.Fatal("expected error on invalid decimal data, got nil")
    }
}