Fix template resolution to support subdirectory paths

[Copilot]

Templates in subdirectories couldn't be referenced by path (e.g., @await PartialAsync("template/topNav")), and filename-only lookups conflicted when multiple templates shared the same name.

Changes

RazorRenderer.cs

• Store templates with both relative path (template/topNav) and filename (topNav) keys
• Add FindTemplate() with multi-strategy lookup: exact match, extension variants, normalized separators
• Update RenderTemplateContent() and RenderContentAsync() to use new lookup

RazorTests.cs

• Add tests for subdirectory resolution, backward compatibility, extension handling, path normalization

Usage

// All now work correctly:
@await PartialAsync("template/topNav")           // subdirectory path
@await PartialAsync("template/topNav.cshtml")    // with extension
@await PartialAsync("Footer")                    // root-level (backward compat)

Previously, only root-level filename lookups worked, and subdirectory templates with duplicate names would conflict.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Inconsistent Partial rendering behavior</issue_title>
<issue_description>User report:

so this is interesting. the docs say i can do this:

@await PartialAsync("topNav")

and i have a template/topNav.cshtml file, however, it wouldn’t build until i fully qualified it:

@await PartialAsync("template/topNav.cshtml")

and after that, instead of loading the topNav, it actually loaded the Default.cshtml, so i have two body contents here:

        @await PartialAsync("template/topNav.cshtml")

        @Raw(Model.Body)

if i remove that topNav template inclusion, i only get one body content.

Acceptance Criteria

• paths should resolve relatively ... so if template/topNav.cshtml is being rendered from another template in that same template folder, it should correctly resolve.
• The 2nd reported issue, of it rendering Default.cshtml instead of the requested topNav.cshtml is obviously incorrect.

First, confirm this behavior by compiling the project, creating a conceptual template, and modifying it as described. Second, fix the issues and add unit tests to match the correct behavior. And lastly, re-compile, boostrap a conceptual template, and re-try to confirm fixed behavior.</issue_description>

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

• Fixes Inconsistent Partial rendering behavior #64

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