Fix icon theme not updating on reload after Dark/Light mode switch

[Copilot]

RevitThemeDetector stores the detected Revit UI theme in static fields, so after switching themes mid-session the stale value persists for the process lifetime — a full Revit restart was required to see correct icons.

Change

Clear the theme cache at the start of each LoadSession() so the active theme is re-queried on every reload:

// SessionManagerService.LoadSession()
_extensionManager?.ClearParserCaches();
RevitThemeDetector.ClearCache();   // ← added

The cache is repopulated on the first IsDarkTheme() call within that same load, so there is no performance regression. After this change, a pyRevit reload is sufficient to pick up a Dark ↔ Light mode switch.

[devloai]

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[jmcouffin]

Fixes #3361

[jmcouffin]

@romangolev @Wurschdhaud @tay0thman what are you thoughts on this? Shall we leave it as is and document it in the reload button tooltip or adjust the behavior following this PR?