feat(fxconfig): add --format flag to namespace list with JSON/YAML output

[pradhyum6144]

Type of change

• New feature
• Improvement (improvement to code, performance, etc)

Description

Adds a --format flag (json|yaml|table) to fxconfig namespace list.
The default table format preserves the existing output exactly for
backward compatibility.

For json and yaml, the command serialises a typed slice with the
following fields per namespace:

Field	Description
name	Namespace identifier
version	Current version number
policy	Raw policy bytes (base64 in JSON)
policyString	Human-readable decoded policy

Policy decoding (policyString) is implemented in the CLI layer:

• ThresholdRule → base64-encoded public key
• MspRule → proto text of SignaturePolicyEnvelope
• Unknown/invalid bytes → hex fallback

The commented-out parsePolicy stub in app/list.go is removed. It
targeted common.SignaturePolicy rather than common.SignaturePolicyEnvelope
(the actual type produced by policydsl.FromString), and used panics for
error handling. The correct implementation now lives in the CLI layer where
display concerns belong.

Additional details (Optional)

Tests added:

• TestNewListCommandRunJSON — validates JSON structure and field presence
• TestNewListCommandRunYAML — validates name and version fields in YAML
• TestNewListCommandRunInvalidFormat — invalid format returns descriptive error
• Updated TestNewListCommandRun to use cmd.SetOut (idiomatic Cobra test pattern)

All unit tests pass: go test ./fxconfig/internal/...
Lint clean: golangci-lint run --new-from-rev=origin/main ./fxconfig/...

Example usage after this change:

# pipe into jq for scripting
fxconfig namespace list --format json | jq '.[].name'

# YAML for human inspection
fxconfig namespace list --format yaml