feat(framework): align FactoryFunc signature with EPP (json.RawMessage → *json.Decoder)

[szedan-rh]

Parent

Sub-issue of #221

Summary

Align IPP's FactoryFunc signature with EPP's to use *json.Decoder instead of json.RawMessage, enabling strict parameter parsing via DisallowUnknownFields.

Context

This is independent of the CycleState → request attributes migration (#288) and can be done in parallel.

Current (IPP):

type FactoryFunc func(name string, parameters json.RawMessage, handle Handle) (Plugin, error)

Target (EPP):

type FactoryFunc func(name string, parameters *json.Decoder, handle Handle) (Plugin, error)

Changes

1. Import FactoryFunc and StrictDecoder from EPP

• Add FactoryFunc = eppplugin.FactoryFunc type alias in epp.go (same pattern as Phase 1)
• Import StrictDecoder from EPP (converts json.RawMessage → strict *json.Decoder)

2. Update all plugin factory implementations

• Change json.Unmarshal(rawParameters, &config) to rawParameters.Decode(&config)
• Handle nil decoder (no parameters) instead of len(rawParameters) == 0
• All 15 in-tree factories in cmd/runner/runner.go need updating

3. Update config loader

• Use StrictDecoder at the call site where raw config parameters are passed to factories

Acceptance criteria

• FactoryFunc imported from EPP via type alias
• All plugin factories use *json.Decoder parameter
• StrictDecoder used at config loading call site
• Unknown fields in plugin config now cause errors (strict parsing)
• make test passes
• make lint passes

[szedan-rh]

PR: #295 raised.

[szedan-rh]

/assign