Add BEFORE_RESPONSE_SENT request phase to the stub-mapping schema

[picimako]

I added the BEFORE_RESPONSE_SENT enum value to the stub-mapping schema as a possible serveEventListener request phase.

The related PR that added support for this was merged back in August, so it was missing from the schema.

References

• Add a serve event listener triggered just before the response is sent #2294
• Add a beforeResponseSent serve event listener hook #2295

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