fix(core): [Callback Errors 5] Handle tracesSampler failures

[adinauer]

PR Stack (Callback Errors)

• fix(core): Handle callback errors #6140
• fix(core): [Callback Errors 1] Report callback failures #6141
• ref(core): [Callback Errors 2] Mark SDK-owned event processors #6162
• fix(core): [Callback Errors 3] Drop failed processor data #6142
• fix(core): [Callback Errors 4] Drop failed breadcrumbs #6143
• fix(core): [Callback Errors 5] Handle tracesSampler failures #6163
• fix(core): [Callback Errors 6] Handle profilesSampler failures #6164
• fix(core): [Callback Errors 7] Handle replay sampling failures #6165
• fix(core): [Callback Errors 8] Report dropped profiles #6166
• fix(integrations): [Callback Errors 9] Guard custom callbacks #6167

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📜 Description

When tracesSampler throws, return an unsampled, unprofiled decision and immediately record callback_error for the transaction and root span. Do not inherit the parent sampling decision or fall back to tracesSampleRate.

A callback returning null still follows the existing parent/static-rate fallback. Explicit sampling decisions still bypass the callback. Error logging, catch types, and profilesSampler behavior are unchanged.

Keep existing downstream sample_rate/backpressure loss reporting. Duplicate reporting is accepted: the same transaction/root span can be counted under both callback_error and the normal sampling reason. No discard-reason state or OpenTelemetry attribute propagation is added.

💡 Motivation and Context

Parent inheritance or a static rate after a sampler failure can send traces that the failed callback intended to exclude. Fail closed on callback errors while preserving deliberate null fallback. Report callback errors directly at the failure boundary rather than adding cross-cutting state to suppress duplicate reports.

• refs: Align callback error handling with other SDKs #6081
• Spec discussion: docs(sdk): Specify callback error isolation for user callbacks sentry-docs#19189

💚 How did you test it?

• ./gradlew spotlessApply apiDump
• Full :sentry:test and :sentry-opentelemetry:sentry-opentelemetry-core:test suites — 3,802 tests passed on this branch.
• API checks passed for core, OpenTelemetry bootstrap, and OpenTelemetry core.
• Covered sampled/unsampled/absent parents, ignored static rates, zero sample-random values, disabled profiling, deliberate null fallback, explicit decisions, and recovery on subsequent calls.
• Verified immediate callback-error reports and retained sampling/backpressure reports, repeated finish, OpenTelemetry root/child accounting, and decisions forwarded through the span factory.
• After merge-forward through fix(integrations): [Callback Errors 9] Guard custom callbacks #6167, 3,814 core/OpenTelemetry tests passed, along with formatting and API checks.

📝 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.
• Public API changes reviewed by another Mobile SDK team member or implemented according to the develop docs spec.

🔮 Next steps

Handle profilesSampler failure semantics separately in #6164. The broader callback-boundary catch-type changes remain deferred to the next major.

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

[sentry]

📲 Install Builds

Android

🔗 App Name	App ID	Version	Configuration
SDK Size	io.sentry.tests.size	8.57.0 (1)	release

⚙️ sentry-android Build Distribution Settings

[adinauer]

This causes double reporting for transactions and spans with reason callback_error and again with sample_rate or backpressure. We discussed internally and are accepting this for now as the alternative would be more complexity.

If we reconsider, we can add a DiscardReason to TracesSamplingDecision. It also needs to be transported into our io.sentry.opentelemetry.SentrySampler.