Warn at codegen time about conflicting names in xmlns

### Description

When there's a conflicting names in a XAML xmlns, the inflators used to pick the First() (but with ordering undefined). #29579 makes the inflation fail, at runtime or XamlC time.

We should be proactive and report a diagnostic warning at the time the code behind for the xaml (xmalg) is generated, so the issue can be reported in the IDE

### Public API Changes

none

### Intended Use-Case

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Start by reading #29579 and tracing the XAML code-generation path that produces the xmalg code-behind, then examine how conflicting names in an xmlns are detected. Confirm the existing runtime and XamlC failure behavior; done means generation emits a diagnostic warning for the conflict so it can be reported in the IDE instead of silently selecting First().

Written by the indexing model from the issue text.