[Blazor] Consider a build-time analyzer for form method=dialog / non-default target instead of the runtime enhanced-navigation warning

## Summary

`src/Components/Web.JS/src/Services/NavigationEnhancement.ts`'s `onDocumentSubmit` handler currently detects two cases where a form cannot be enhanced, and logs a runtime `console.warn` for each:

```ts
const method = event.submitter?.getAttribute('formmethod') || formElem.method;
if (method === 'dialog') {
console.warn('A form cannot be enhanced when its method is "dialog".');
return;
}

const target = event.submitter?.getAttribute('formtarget') || formElem.target;
if (target !== '' && target !== '_self') {
console.warn('A form cannot be enhanced when its target is different from the default value "_self".');
return;
}
```

Both are genuine functional issues (the form silently falls back to a full page load instead of being enhanced), not style preferences, so a warning is justified. However, both conditions are driven by static attributes on the `` / submit button (`method` / `formmethod`, `target` / `formtarget`) that are almost always known at compile time from the `.razor` markup.

## Ask

Investigate whether a build-time Roslyn analyzer (in `Microsoft.AspNetCore.Components.Analyzers`) can detect these same two cases directly from `.razor` markup: a `` (or ``) with a literal `method="dialog"`, or a literal `target` / `formtarget` value other than `""` / `"_self"`, combined with enhanced navigation being enabled (the default, i.e. not opted out via `data-enhance-nav="false"` or similar).

If feasible, this would let developers catch the misconfiguration in the editor / at compile time instead of only discovering it at runtime via the console warning, the same motivation discussed for the Virtualize spacer-element case.

## Constraint

The analyzer can only fire when the value is a static literal. Dynamic or data-bound `method` / `target` / `formmethod` / `formtarget` values (e.g. `method="@someExpression"`) cannot be analyzed statically, so the runtime `console.warn` must remain as the fallback for those cases.

Start in Microsoft.AspNetCore.Components.Analyzers and review the existing Virtualize spacer-element analyzer approach. Compare its possible Razor markup inputs with the onDocumentSubmit handler in src/Components/Web.JS/src/Services/NavigationEnhancement.ts. Done means determining whether static method/target values can be diagnosed when enhanced navigation is enabled, while dynamic values remain covered by the runtime warning.

Written by the indexing model from the issue text.