Colocate explicit DTO generation with SelectExpr files

[Copilot]

Explicit DTOs were always emitted into a global generated file, making it hard to see the SelectExpr replacement context. DTOs under hashed namespaces may stay shared; playground code can remain single-file.

• Generation flow

  • Per-file/group DTO collection and deduplication; hash-namespace DTOs still emitted globally.
  • Expression snippets now embed co-located DTO code when present.
  • Hash-namespace detection centralized via a prefix constant.

• Test

  • Added regression ensuring explicit DTOs appear in the same generated expression file and not in the global DTO file.

Example (conceptual)

// Before: DTO emitted only in GeneratedDtos.g.cs
query.SelectExpr<SimpleDto>(x => new { x.Id });

// After: DTO emitted alongside GeneratedExpression_*.g.cs for that file/group
// Hash-namespaced auto DTOs remain in the shared DTO file.

Original prompt

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This section details on the original issue you should resolve

<issue_title>feat: It would be convenient if explicit DTO classes were created within the same file as the intercepted SelectExpr.</issue_title>
<issue_description>Although the improvements made in PR #240 have resolved declaration duplication, the current implementation generates all classes in a single file and performs filtering in the process. However, due to the nature of the Interceptor, there is currently no way to directly view the replacement code for SelectExpr. As a stepping stone, it would be useful to generate DTO classes in the same file as SelectExpr.
Specifically, explicit DTOs should be in the same file as the generated SelectExpr method. For (auto-inferred) DTO classes generated in the namespace Linqraft_Hash1234, it is acceptable to generate them in separate files from the perspective of duplication management.
For code generated in the Playground, it is still acceptable to generate it in a single file.</issue_description>

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

• Fixes feat: It would be convenient if explicit DTO classes were created within the same file as the intercepted SelectExpr. #247

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