fix(pipeline): reject partially-consumed input in JSONWithVarExprs

[ozpool]

Description

Closes #21768.

The pipeline JSONWithVarExprs getter (core/services/pipeline/getters.go) uses a streaming json.Decoder that reads only one top-level token. For a literal Ethereum address in an ethtx task's from field the decoder reads the leading 0 as a complete JSON number, returns int64(0), and discards the rest of the address:

perform [type="ethtx"
         from="0xAaAa...AAAA"
         ...]

fatal=true
run.FatalErrors=["from: AddressSliceParam: cannot convert int64: bad input for task"]

Because JSONWithVarExprs successfully returns int64(0), the next getter in the From chain (NonemptyString) is never tried, and AddressSliceParam.UnmarshalPipelineParam hits its default branch and errors out.

Issue author Beam already traced this in the report; root cause is that the decoder doesn't validate that it consumed the entire input.

Fix

Tighten JSONWithVarExprs to require the decoder to consume the full input. After jd.Decode(...) returns, call jd.Token() once and require io.EOF; anything else means there's trailing data, which the getter rejects with ErrBadInput so the next getter in the From chain (e.g. NonemptyString) gets its turn.

if _, err := jd.Token(); !stderrors.Is(err, io.EOF) {
    return nil, errors.Wrapf(ErrBadInput, "trailing data after JSON value; js: %s", string(replaced))
}

Wire-observable behavior:

• Valid JSON (objects, arrays, complete numbers, strings, null) — unchanged. All existing TestGetters_JSONWithVarExprs cases still pass.
• Literal 0xABCD... and similar partially-consumable inputs — now correctly fall through to the next getter, so from = "0x..." resolves via NonemptyString and the ethtx task succeeds.

Tests

Added regression cases to the existing TestGetters_JSONWithVarExprs table:

• 0x1234567890abcdef1234567890abcdef12345678 (full 20-byte address)
• 0xdeadbeef
• 123abc (number prefix + garbage suffix)
• {"a":1} extra (valid JSON + trailing junk)

All four assert pipeline.ErrBadInput, matching the next-getter-falls-through contract.

$ go test -count=1 -run "TestGetters_JSONWithVarExprs" ./core/services/pipeline/
ok   github.com/smartcontractkit/chainlink/v2/core/services/pipeline   2.142s

DB-backed tests in this package (TestETHTxTask, TestETHCallTask) require CL_DATABASE_URL and a Postgres test database, which I don't have set up locally; happy to add a focused TestETHTxTask_FromLiteralAddress if a maintainer can either point me at the right setup or run those checks in CI.

Diff

 .changeset/fix-ethtx-from-literal-address.md    | +9
 core/services/pipeline/getters.go               | +11
 core/services/pipeline/getters_test.go          | +8