RTECO-2020 - Improve agent plugins tests

[udaykb2]

agent_plugins_test: add native harness verification with improved error handling

• Add assertPluginsInstalledNatively() to verify plugins via harness native CLI (claude/codex plugin list --json, cursor/vscode manifest files)
• Add verifyIsolatedHome() to verify HOME env var isolation works correctly
• Add 30-second timeout to CLI commands to prevent test hangs
• Parameterize VSCode repo path instead of hard-coding for test flexibility
• Include stderr context in JSON parse error messages for better debugging
• Improve type assertion error messages to show actual types received
• Fix platform-specific temp path check to work cross-platform (Linux/macOS/Windows)
• Add input parameter validation to catch test misconfiguration early
• Add context import for timeout support
• Update 16 fixture definitions to include all 4 harnesses (claude/codex/cursor/vscode)

This improves test reliability, debuggability, and cross-platform compatibility.

• All tests have passed. If this feature is not already covered by the tests, new tests have been added.
• The pull request is targeting the master branch.
• The code has been validated to compile successfully by running go vet ./....
• The code has been formatted properly using go fmt ./....

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