Include namespace in generated source filename to prevent duplicate hint-name collisions

[Copilot]

When two classes share the same name across different namespaces (e.g. MyNamespace1.WelcomeWizardPage and MyNamespace2.WelcomeWizardPage), all source generators produced the same hint name, causing a Roslyn exception on AddSource.

Changes

New shared utility: SourceFileNameHelper

• GetGeneratedFileName(namespaceName, className) replaces the per-generator ad-hoc filename logic across all five generators
• Returns {Namespace_With_Dots_As_Underscores}_{ClassName} when combined length ≤ 100 chars
• Falls back to {8-char-FNV-1a-hash-of-namespace}_{ClassName} for very long namespaces — hash is deterministic, preserving incremental build caching

// MyNamespace1.WelcomeWizardPage → MyNamespace1_WelcomeWizardPage_ViewModelConstructors.g.cs
// MyNamespace2.WelcomeWizardPage → MyNamespace2_WelcomeWizardPage_ViewModelConstructors.g.cs

Generators updated

All five generators (ViewModelConstructors, ViewConstructors, BehaviorConstructors, MarkupExtensionConstructors, ValueConverterConstructors) now delegate filename construction to SourceFileNameHelper.

Also corrects a pre-existing bug in ValueConverterConstructorsSourceGenerator where it was emitting _BehaviorConstructors.g.cs as the file suffix instead of _ValueConverterConstructors.g.cs.

Tests

• SourceFileNameHelperTests — 7 unit tests covering dot replacement, short/long name thresholds, determinism, and uniqueness across namespaces
• ViewModelCtorGeneratorTests.duplicate_class_names — integration test reproducing the exact issue: two WelcomeWizardPage ViewModels in different namespaces produce distinct, non-clashing hint names
• All 15 existing Verify snapshot files updated to reflect the new namespace-prefixed hint names

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