Plan viewer: the empty Server Context card no longer points to the repro script

[erikdarlingdata]

Fixes #4623. Part of #4511.

Why

The empty Server Context card said "Run Repro Script to capture server context", copied from PerformanceStudio. In PerformanceMonitor the card fills only from the ServerMetadata a host sets from its own store; running the repro script never fills it.

Hosts that set ServerMetadata: the Lite server tab, the Lite plan window and drill-down collector, the Darling Viewer server tab, the actual-plan flow, and the procedure, query-stats, Query Store and wait drill-down windows. It stays null for a pasted plan, a file open, and analyze_plan_xml.

What changes

• New constant ServerContextCard.EmptyText: "Server context is shown for plans opened from a monitored server's data."
• The XAML literal is removed; ShowServerContext() sets the text from the constant when there are no rows.

Lite gets this too (shared control). PM-ahead: the wording differs from PerformanceStudio on purpose, because its text is false here.

Test plan

• Viewer4623Tests (2 pins): the constant's exact text with no "Repro"; a source pin that the XAML has no literal and the code uses the constant.
• RED on dev: compile failure (ServerContextCard.EmptyText doesn't exist), and the source pin would also fail on the old XAML literal.
• GREEN: Total: 83, Failed: 0 (Viewer4623Tests, DocCommentHygieneTests, CommentFilterAdoptionTests); Total: 68, Failed: 0 for every *Census* class plus McpPayloadContractCensusTests. Darling.Tests build: 0 warnings, 0 errors.
• Not run: Lite and Viewer builds (only a text/constant change in shared files); CI decides.
• Screenshot plan: open a pasted plan or a file in the Darling Viewer or Lite; the first insights card shows the new sentence, muted. Open a plan from a server tab: the card shows rows.

CHANGELOG

None: the Server Context card is new in this release (#4614), so its empty-state text never shipped.