Support Eloverblik-style specs — operationId synthesis, spec validation, and flattened endpoint namespaces

[davidkallesen]

Summary

Adds support for generating clients from the Eloverblik APIs, which exposed several
gaps in the generator: specs without operationId, missing quoting on string defaults,
and an endpoint file/namespace layout that produced excessive nesting and one-file folders.

Two new sample projects (EloverblikCustomerApiClient, EloverblikThirdPartyApiClient)
are included as living regression coverage, and the existing MontaClient sample was
moved under a shared sample/ThirdParty-Clients/ folder.

Changes

Specification handling

• Synthesize operations without operationId — the extractor now derives a stable
  name from the HTTP method and path, and generates the corresponding parameter records
  instead of skipping the operation.
• Improved spec validation — OpenApiDocumentValidator reports a clear diagnostic
  when operationId is missing rather than failing silently downstream.
• Fixed default string values — string defaults are now emitted quoted.

Endpoint layout (server)

• Consolidated endpoint mapping — instead of one EndpointDefinitionExtensions.cs
  per path segment (each holding a single Map{Segment}Endpoints() method), a single
  root Endpoints/EndpointDefinitionExtensions.cs now contains one method per segment.
• Flattened endpoint definitions — endpoint definition classes are already
  segment-prefixed (EmployeesEndpointDefinition), so they now live directly in
  Endpoints/ under the root *.Generated.Endpoints namespace instead of one folder
  per segment. Folder structure and namespace remain aligned, and the cross-namespace
  using blocks in both extension files are gone.
• IEndpointDefinition is emitted once as its own shared file.

Namespace/file alignment

• Centralized segment resolution via PathSegmentHelper.ResolveEffectivePathSegment(...)
  so a redundant path segment collapses consistently across the Roslyn generators and
  the CLI CodeGenerationService.
• Added NamespaceBuilder.ToFileName(...) and aligned generated file names with their
  namespaces across the client, server, and CLI paths.

Samples

• Added sample/ThirdParty-Clients/EloverblikCustomerApiClient and
  sample/ThirdParty-Clients/EloverblikThirdPartyApiClient.
• Moved MontaClient into sample/ThirdParty-Clients/.

Cleanup

• Switched to is null / is not null throughout.

Validation

• dotnet build — succeeded, 0 warnings, 0 errors.
• Integration tests — 505 passed, 0 failed from a clean output directory.
• Verify snapshots under test/Scenarios/** regenerated to match the new layout;
  the diffs are limited to file relocation and removed redundant using directives.

Breaking changes

The generated server endpoint namespace changed from {Project}.Generated.{Segment}.Endpoints
to {Project}.Generated.Endpoints for endpoint definition classes. Consumers referencing
these types directly will need to update their using directives. The public
MapEndpoints() entry point is unchanged.