fix(pipeline): accept literal address in ethtx from field

[PrazwalR]

Motivation

Fixes #21768. An ethtx pipeline task with a literal Ethereum address in the
from field — from="0x8829..." — always fails with:
from: AddressSliceParam: cannot convert int64: bad input for task

Every address starts with 0x, so no literal from address works. Only the
JSON-array form (from="[ \"0x..\" ]") and var-expr form ($(fromAddr)) work
today.

Root cause

from is resolved by this getter chain (core/services/pipeline/task.eth_tx.go):

From(VarExpr(t.From, vars), JSONWithVarExprs(t.From, vars, false), NonemptyString(t.From), nil)
ResolveParam only continues to the next getter on ErrParameterEmpty.

In JSONWithVarExprs, json.Decoder.Decode reads a single JSON value and
silently ignores trailing bytes (unlike json.Unmarshal). Fed 0x8829..., it
reads the leading 0 as a complete JSON number, succeeds, and drops the rest;
ReinterpretJSONNumbers turns it into int64(0). The getter "succeeds" with
int64(0), the chain stops before NonemptyString, and AddressSliceParam
hits its default branch, returning cannot convert int64.

Fix
JSONWithVarExprs — reject input the decoder only partially consumed,
returning ErrParameterEmpty (not ErrBadInput) so ResolveParam falls
through to NonemptyString.
AddressSliceParam — accept a single literal address in addition to a
JSON array, since a bare 0x... is not valid JSON on its own.
Both changes are needed: (1) alone still fails, because NonemptyString then
passes a bare address string that (2) is required to parse.

Tests
TestGetters_JSONWithVarExprs: added partial-consumption cases (literal
address, 123abc, {} extra, [1,2] junk, etc.)
TestAddressSliceParam_UnmarshalPipelineParam: added single-literal
(string/[]byte) and garbage-string cases
TestAddressSliceParam_ResolveFromGetterChain_Issue21768: new DB-free test
driving the exact production from resolver chain end-to-end
TestETHTxTask: added a literal-from end-to-end case
Verified with a negative control: reverting the fix makes the new tests fail
with the original cannot convert int64 error; restoring it makes them pass.
go vet ./core/services/pipeline/ is clean.

Test plan
done
go test ./core/services/pipeline/ -run 'TestGetters|Param_UnmarshalPipelineParam|ResolveFromGetterChain_Issue21768' -v
done
go vet ./core/services/pipeline/
not done
TestETHTxTask full suite (requires CL_DATABASE_URL, run in CI)

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