fix!(core): Make HubSwitchGuard !Send to prevent thread corruption

[szokeasaurusrex]

Description

HubSwitchGuard manages thread-local hub state but was Send, allowing it to be moved to another thread. When dropped on the wrong thread, it could corrupt that thread's hub state instead of restoring the original thread.

This PR makes HubSwitchGuard !Send by adding PhantomData<MutexGuard<'static, ()>> while keeping it Sync. The type system now prevents the guard from being moved across threads at compile time.

To ensure guards are always dropped on the originating thread, sentry-tracing now stores them in thread-local storage keyed by span ID rather than in span extensions.

Issues

• Fixes HubSwitchGuard should not be Send #943
• Resolves RUST-130

[linear]

RUST-130 HubSwitchGuard should not be Send

RUST-132 Entering the same span several times causes esoteric behavior

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[lcian]

Looks good to me.

[szokeasaurusrex]

This current implementation is still flawed; if the same span is re-entered in the same thread (possible in async contexts), the second entry overwrites the original HubSwitchGuard, corrupting the behavior. I am working on a fix

[lcian]

@szokeasaurusrex it rewrites the original guard with the same one I think, so what's the issue there?

[lcian]

I've tested some scenarios manually and didn't find any issues. I assume you're also testing manully.
Would be nice to add these as automated tests as well.

[szokeasaurusrex]

@lcian Codex AI agent identified the potential issue here as I was working on #946. I have a test locally which reproduces the behavior. Will commit it with a more detailed explanation of the problem tomorrow 👍

I suppose it's possible that Codex is wrong and the local test is doing something which is not supposed to ever happen (the scenario is admittedly a bit contrived), but in any case, I think it's worth it to investigate properly. So, that is what I'm doing now. I will let you know if I need any assistance