Fixes #3985. Cache URI validation in Link to avoid revalidation on every redraw

[Copilot]

Link.OnDrawingText called Uri.IsWellFormedUriString on every redraw cycle. In TUI applications with many Link views or frequent redraws (layout invalidation, animations, refresh), this is redundant parsing of the same string.

Changes

• _isUrlValid backing field added alongside _url, initialized to false (matches the DEFAULT_URL = "" default which is not a valid absolute URI)
• Validation moved to SetUrl — computed once when Url changes, cached in _isUrlValid
• OnDrawingText uses _isUrlValid — eliminates the per-draw Uri.IsWellFormedUriString call
• Url XML doc updated to reflect validation happens at set-time, not draw-time

// Before: validated on every redraw
if (!Uri.IsWellFormedUriString (Url, UriKind.Absolute)) { ... }

// After: validated once in SetUrl, cached
_url = value;
_isUrlValid = Uri.IsWellFormedUriString (value, UriKind.Absolute);

// OnDrawingText uses the cache
if (!_isUrlValid) { ... }

Behavior is identical — empty/invalid URLs still render with VisualRole.Disabled and suppress OSC 8 sequences. All existing LinkTests pass.

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Original prompt

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This section details on the original issue you should resolve

<issue_title>Potential perf issue : Link URL validation moved to draw time may cause performance issues</issue_title>
<issue_description>Hi there,

Link view PR was merged with some adaptation.

URL validation moved to draw time — Url accepts any string in the setter (with full CWP event support via UrlChanging/UrlChanged). Validation happens in OnDrawingText: invalid URLs render with the Disabled visual role and don't emit OSC 8 sequences.

One small point of attention regarding the URI validation.

I noticed that Uri.IsWellFormedUriString is currently evaluated during rendering. While this works functionally, it means the URI will be revalidated every time the view is redrawn.

In most cases this probably has negligible impact, but since rendering can occur frequently in a TUI (e.g., layout invalidation, refresh cycles, animations, etc.), repeatedly parsing the same URI could become unnecessary work if many Link views are present.

An alternative approach could be to validate the URI when the Url property changes and cache the result, keeping the render path as lightweight as possible.

Of course this is not a blocker, just a small performance consideration depending on how often the control might be redrawn.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Potential perf issue : Link URL validation moved to draw time may cause performance issues #4832

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