Surface Query Store plan-load failures instead of silent 'No Plan Loaded'

[erikdarlingdata]

Problem

Loading a plan from the Query Store grid could blank to "No Plan Loaded" while the QS grid itself vanished, with no explanation.

Root cause

• PlanViewerControl.LoadPlan treated a failed parse identically to an empty plan: ShowPlanParser.Parse never throws — it records failures in ParsedPlan.ParseError and returns a zero-statement plan — but the desktop app never read ParseError, so any parse failure rendered as a bare "No Plan Loaded".
• AddPlanTab unconditionally added and selected the new plan tab even when the load failed, navigating away from the grid to that blank tab.

Commit e8e5a21 (parser hardening) was the regression vector: it wrapped the parser tree-walk in a catch-all, converting what used to be a visible crash on a bad plan into a silent zero-statement ParseError — i.e. exactly this symptom.

Fix

• LoadPlan returns bool and exposes LastLoadError, distinguishing blank/NULL plan XML, a parse error (surfaced verbatim in the empty-state panel), and parsed-but-no-statements.
• AddPlanTab no longer switches away on failure — it keeps the Query Store grid active and leaves a persistent status line explaining why.
• Clear() resets the empty state so a reused viewer can't show a stale error (caught by the Avalonia gotcha review).

The four direct LoadPlan callers (File→Open, paste, estimated/actual execution) ignore the new return value and keep their prior behavior; they additionally benefit from real error text in the empty state.

Note

This makes failures visible and non-destructive; it does not alter the parser. If a specific plan still fails, the on-screen "Parse error: …" now names the cause so the underlying parser issue can be fixed separately.

Testing

• dotnet build PlanViewer.App: 0 errors.
• App launches clean; empty state renders with no Avalonia binding errors.
• Avalonia gotcha review: clean (no null-element risk, no event-handler leak, no orphaned MCP session — failed loads never register one).

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