Copilot plugin and marketplace discovery hide native command failures

Peer-review finding

Reviewed snapshot: 5c38677469756c69f86678c0b907d82e15d0a539.
The reviewer and parent independently reproduced this against unchanged source.
Only fresh synthetic session homes and a fixed non-mutating native stub were
used; no real conversation history, user configuration or Copilot process was
accessed. Do not run destructive reproduction commands against real sessions.

Severity: Medium. Confidence: 10/10.
Suggested existing labels: bug, module:copilot.

Locations: modules\Shmuelie.Copilot\Plugins.ps1:26-29
(Get-CopilotPlugin); modules\Shmuelie.Copilot\Marketplaces.ps1:23-26
(Get-CopilotMarketplace) and 143-146 (Get-CopilotMarketplacePlugin).

These discovery commands capture combined native output and emit only lines
matching their successful-list patterns. They never check the native exit code,
and discarded diagnostics are not surfaced on PowerShell's error stream.
Consequently discovery failure is indistinguishable from an empty installation
or marketplace; dependent pipelines can quietly do no work.

Minimal controlled-native reproduction:

Use a scratch .cmd as the only resolved native boundary:

@echo off
echo Error: synthetic native discovery failure
exit /b 17

With $PSNativeCommandUseErrorActionPreference = $false (the standard opt-out
behavior), redirect the module's command resolver to that stub and invoke each
command with -ErrorAction Stop -ErrorVariable errors. For marketplace browsing,
pass -Name synthetic.

Actual for all three: Native exit code 17, zero result objects, zero error
records, and no caught exception. The diagnostic disappears. No real marketplace,
credentials, or user configuration was accessed.

Expected: Native failure is reported as a PowerShell error with actionable
diagnostics, distinguishable from an empty successful response. -ErrorAction Stop must be effective for callers.

Acceptance criteria: Capture/check native status before successful-output
parsing and propagate diagnostics. Add regressions for nonzero exits and successful
empty lists for all three readers. Ensure dependent install/register/update
pipelines cannot treat failed discovery as a successful empty inventory.

Related pattern: #259 covers separate DSC resource wrappers. This issue covers the canonical Shmuelie.Copilot discovery commands and is not the DSC marketplace argument issue #261.

Start with Get-CopilotPlugin in modules\Shmuelie.Copilot\Plugins.ps1 and Get-CopilotMarketplace plus Get-CopilotMarketplacePlugin in Marketplaces.ps1. Run the controlled .cmd stub through the command resolver, including -Name synthetic for marketplace browsing, then add regressions covering nonzero exits and successful empty lists. Done means native diagnostics become PowerShell errors, -ErrorAction Stop works, and dependent pipelines do not treat failed discovery as empty.