fix(server,cli): add JSON error code/suggestion; warn on PluginService type mismatch

[lukgoph]

Summary

• Scope: Issue API/CLI Error Handling Improvements #90 — API/CLI error handling improvements
• Impact: Machine-readable errors in CLI and better router observability
• Risk: Low; additive fields and logs only (no behavior change)

CLI (JSON error output)

• In JSON mode, the error payload now includes code and suggestion in addition to existing success:false and error fields
• code: produced via plugin.ErrorCode(err)
• suggestion: produced via plugin.GetSuggestion(err)
• Backward compatible: existing fields are preserved; schema is extended

Server/Router (observability)

• When mounting plugin routes:
  • Correct type → info: "mounting plugin API routes"
  • Wrong type → warn: "PluginService type assertion failed … NOT mounted" + actual_type
  • Nil service → info: "PluginService not provided – skipping plugin API routes"

Tests

• CLI: Added/updated tests to assert presence and correctness of code and suggestion across representative errors (NotFound, InvalidInput, Unavailable)
• Router: Added tests for nil/wrong/correct PluginService types; assert route behavior and corresponding log messages

Validation

• make test && make validate pass locally

Closes

• Resolves API/CLI Error Handling Improvements #90

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[weissarc]

LGTM. JSON error output gains code/suggestion (additive, backward-compatible). Router logs for PluginService mount are clear and tested. CI green; ready to merge.