feat(core): [Unhandled Sessions 1] Add Unhandled session state and pending marker

[buenaflor]

PR Stack (Unhandled Sessions)

• feat(core): Unhandled Sessions #5915 (collection)
• feat(core): [Unhandled Sessions 1] Add Unhandled session state and pending marker #5916
• fix(core): [Unhandled Sessions 2] Make session cache writes session-id aware #5917
• feat(android): [Unhandled Sessions 3] Add internal non-terminating envelope capture #5918

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📜 Description

Adds the unhandled session status to the session model.

• Session.State.Unhandled — the protocol status for an unhandled error that did not terminate the process.
• A pendingUnhandled marker on Session (pending_unhandled when serialized) that records "this session saw an unhandled error" without ending the session or changing its status while it is still alive.
• end() finalizes a session carrying the marker as Unhandled instead of Exited. Crashed and Abnormal keep taking precedence, and update(Crashed, ...) clears the marker so a real crash always wins.
• clone() and the (de)serializer carry the marker; it is omitted from JSON when unset.

Nothing sets the marker yet — this PR is inert on its own. It is the model layer for the rest of the stack.

💡 Motivation and Context

Hybrid runtimes such as Flutter report handled=false exceptions that do not kill the process. Today those go through the terminating capture path, which marks the session crashed and starts a replacement session even though the app keeps running, incorrectly lowering crash-free session rates.

The session protocol has had a dedicated status for exactly this case since 1.6.0: unhandled — "an unhandled error occurred but the process did not terminate". Relay accepts it (SessionStatus::Unhandled → "unhandled").

💚 How did you test it?

New SessionTest covering the marker's atomic update, terminal-state precedence (Crashed/Abnormal win), clone(), and serialization round-trips. PreviousSessionFinalizerTest covers recovery of a previous session carrying the marker, including a native crash escalating it to crashed. SessionSerializationTest covers the JSON round-trip.

📝 Checklist

• I added GH Issue ID & Linear ID
• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• I updated the wizard if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

🔮 Next steps

Persisting the marker across process death and exposing the capture API, in the following PRs in this stack.

#skip-changelog

⚠️ Merge this PR using a merge commit (not squash). Only the collection branch is squash-merged into main.

[github-actions]

	Messages
📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

Generated by 🚫 dangerJS against e62793f

[sentry]

📲 Install Builds

Android

🔗 App Name	App ID	Version	Configuration
SDK Size	io.sentry.tests.size	8.52.0 (1)	release

⚙️ sentry-android Build Distribution Settings

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	341.96 ms	461.54 ms	119.58 ms
Size	0 B	0 B	0 B

[buenaflor]

Not actually merged into main. This PR was auto-closed when the collection branch feat/unhandled-sessions was accidentally fast-forwarded to the tip of the stack, which made GitHub consider every stack branch merged into its base.

The collection branch has been restored to its empty commit and the stack rebuilt unchanged. This PR continues as #5919.