Fix duplicate client/operation names when operation IDs contain multiple underscores

[Copilot]

MultipleClientsFromOperationIdOperationNameGenerator used the second-to-last underscore segment as client name and last segment as operation name. Any two operation IDs sharing the same suffix (e.g. Orders_items_get and Products_items_get) mapped to the same (client="items", operation="get") pair, producing duplicate generated classes and methods.

Changes

• GetClientName: switched from second-to-last segment to everything before the first _
• GetOperationName: switched from LastIndexOf('_') to IndexOf('_') — returns everything after the first _

Operation ID	Old client → operation	New client → operation
OperationId_Test	OperationId → Test	OperationId → Test
Orders_items_get	items → get	Orders → items_get
Products_items_get	items → get ⚠️ duplicate	Products → items_get ✅

Since OpenAPI mandates globally unique operation IDs, splitting at the first _ guarantees that no two operation IDs can ever produce the same (client, operation) pair. The fix also propagates to MultipleClientsFromFirstTagAndOperationNameGenerator, which inherits GetOperationName from the base class.

Tests

• Updated existing test expectations to match the new behavior
• Added explicit operation-name tests for MultipleClientsFromOperationId
• Added regression tests asserting that distinct operation IDs always yield distinct (client, operation) pairs

Original prompt

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This section details on the original issue you should resolve

<issue_title>Underscores - the source of all evil when used, but should work out of the box.</issue_title>
<issue_description>We have a customer openapi.json which we query at runtime to generate our bindings.
In our case we are getting duplicate classes, due to the standard implementation using the underscore as separator, and only giving back the last chunk, which is obviously bad.

Example: https://github.com/RicoSuter/NSwag/blob/master/src/NSwag.CodeGeneration/OperationNameGenerators/MultipleClientsFromOperationIdOperationNameGenerator.cs#L108

This also applies to
#4178 #3450 #1749 #596

Neither we, nor the customer can change those definition file. Manually iterating the definition for replacement is also a huge pain, as we never know where stuff is referenced. And even if we manage to find all references, we cannot rely on removing the underscore, as this might actually generate duplicate identifiers.

• Having an underscore in an OperationId should work.
• Having an underscore in a schema name should also work.

I would suggest using "/" as separator char, as it is invalid anyway, but an "_" underscore is very likely to occur.</issue_description>

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

• Fixes Underscores - the source of all evil when used, but should work out of the box. #4243

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Pull request overview

This PR fixes a long-standing bug in MultipleClientsFromOperationIdOperationNameGenerator where operation IDs with multiple underscores (e.g., Orders_items_get and Products_items_get) would map to duplicate (client, operation) pairs, causing duplicate generated classes and methods. The fix simplifies the splitting strategy to always use the first underscore as the delimiter — everything before it becomes the client name, everything after becomes the operation name. Since OpenAPI mandates globally unique operation IDs, this guarantees uniqueness of (client, operation) pairs.

Changes:

• GetClientName: Returns everything before the first underscore (instead of the second-to-last segment)
• GetOperationName: Returns everything after the first underscore (instead of the last segment)
• Test updates and additions to cover the new behavior and regression-test uniqueness guarantees

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/NSwag.CodeGeneration/OperationNameGenerators/MultipleClientsFromOperationIdOperationNameGenerator.cs	Simplified GetClientName and GetOperationName to split on the first underscore instead of the last
src/NSwag.CodeGeneration.Tests/CodeGenerationTests.cs	Updated existing test expectations to match new behavior, added operation-name tests for MultipleClientsFromOperationId, and added regression tests asserting uniqueness of (client, operation) pairs

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