fix(grpc): add isolation_scope to async server interceptor

[robinvd]

Description

Fixes #5894

The async gRPC server interceptor was missing isolation_scope() wrapping, causing scope state to leak between concurrent requests. The sync interceptor already had this. This PR adds the missing isolation scope to the async unary_unary handler and adds a test proving request isolation.

Issues

• resolves: gRPC aio ServerInterceptor missing isolation_scope per request #5894

Reminders

• Please add tests to validate your changes, and lint your code using tox -e linters.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

[github-actions]

Semver Impact of This PR

🟢 Patch (bug fixes)

📋 Changelog Preview

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New Features ✨

• (ci) Cancel in-progress PR workflows on new commit push by joshuarli in #5994

Bug Fixes 🐛

• (grpc) Add isolation_scope to async server interceptor by robinvd in #5940

Internal Changes 🔧

• Set explicit base-branch for codecov action by ericapisani in #5992

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[ericapisani]

Thanks for opening this PR @robinvd !

This PR adds the missing isolation scope to the async unary_unary handler and adds a test proving request isolation.

I'm not seeing the test mentioned in your description in this change set - was it potentially missed in the commit that was made?