Add support for configurable Plan Preview behavior in piped

[rawadhossain]

What this PR does:
Adds a configurable planPreview section to piped config (PipedSpec in both pkg/config and pkg/configv1).

These values are validated and then passed to planpreview.NewHandler using existing options:

• WithWorkerNum
• WithCommandQueueBufferSize
• WithCommandCheckInterval
• WithCommandHandleTimeout

The configuration is applied in both piped entrypoints (pkg/app/piped/cmd/piped and pkg/app/pipedv1/cmd/piped).

Also includes tests to ensure:

• YAML parsing works correctly
• Invalid (negative) values fail validation

Why we need it:
Currently plan preview behavior can only be tuned internally (mainly in tests).
This change allows to configure things like worker count, queue size, and timing directly from piped config.

Fixes Issue #5916

Does this PR introduce a user-facing change?:

• How are users affected by this change:
  • Users can optionally add a spec.planPreview block in piped YAML:
    • workerNum
    • commandQueueBufferSize
    • commandCheckInterval
    • commandHandleTimeout (duration format like 5s, 10m)
  • If not provided, existing default behavior remains unchanged
• Is this breaking change: No
• How to migrate (if breaking change): N/A