fix(pipeline): allow literal ethtx from addresses

[JasonColapietro]

Summary

• allow ethtx task from fields to use a single literal Ethereum address
• preserve variable-expression, JSON-array, and existing fallback handling
• reject trailing JSON data instead of silently accepting the first value
• add focused parser, getter, and task regression coverage

Root cause

The from resolver tried variable and JSON parsing before its string fallback. A bare 0x... address is not valid JSON and therefore could not be resolved as an address slice.

Impact

Pipeline specs can pin an ethtx task to one literal address without wrapping it in a JSON array. Existing variable and array forms remain supported.

Validation

• gofmt -d on all changed Go files
• go test ./core/services/pipeline -run '^(TestGetters_JSONWithVarExprs|TestETHTxFromGetters_LiteralAddress)$' -count=1
• git diff --check

The broader TestETHTxTask table requires CL_DATABASE_URL; it was not runnable in the local environment and is left to CI.