fix(core): drain buffers in flush() when there is no transport

[sbs44]

Client.flush() short-circuited before emitting the 'flush' event when the client had no transport (e.g. no DSN was configured), leaving the weight-based flushers for logs and metrics with idle setTimeout handles that never resolved. Move emit('flush') above the !transport early return so the setupWeightBasedFlushing listeners on 'flushLogs' / 'flushMetrics' drain their buffers and clear the idle timer on every flush() call.

We originally hit this in Supabase Deno edge function tests where a shared harness calls Sentry.init({ dsn: '', enableLogs: true }) and records metrics per request, then calls Sentry.flush(2000) in the request finalizer. The Deno test sanitizer surfaces the unresolved setTimeout as a leaked op:

error: Leaks detected:
  - 2 timers were started in this test, but never completed.
    at setTimeout (node:timers:14:10)
    at .../@sentry/core/10.48.0/build/esm/client.js:108:9
    at DenoClient.emit (.../@sentry/core/10.48.0/build/esm/client.js:572:17)

Behavior change

Any subscriber registered via client.on('flush', …) will now fire when flush() is called on a transport-less client. I verified the in-repo subscribers all handle this safely:

• setupWeightBasedFlushing → _INTERNAL_flushLogsBuffer / _INTERNAL_flushMetricsBuffer --> client.sendEnvelope, which short-circuits on !this._transport (client.ts:1139)
• SpanBuffer.drain() (spanBuffer.ts:86) — builds envelopes and hands them to the same sendEnvelope guard
• httpServerIntegration --> flushPendingClientAggregates --> client.sendSession... also routed through sendEnvelope
• otlpIntegration --> _spanProcessor?.forceFlush(): OTLP has its own exporter independent of the Sentry transport

Integration authors with custom 'flush' listeners that previously assumed a transport was present may want to audit their handlers.

Tests

Added two regression tests (one for logs, one for metrics) that fail on develop and pass with the fix. Both assert the observable end state (the buffer is drained via 'flushLogs' / 'flushMetrics') rather than relying on an indirect clearTimeout spy.

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Closes #20206

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Semver Impact of This PR

🟢 Patch (bug fixes)

📋 Changelog Preview

This is how your changes will appear in the changelog.
Entries from this PR are highlighted with a left border (blockquote style).

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New Features ✨

Core

• Add enableTruncation option to Google GenAI integration by andreiborza in #20184
• Add enableTruncation option to Anthropic AI integration by andreiborza in #20181
• Add enableTruncation option to LangGraph integration by andreiborza in #20183
• Add enableTruncation option to LangChain integration by andreiborza in #20182
• Add enableTruncation option to OpenAI integration by andreiborza in #20167
• Export a reusable function to add tracing headers by JPeer264 in #20076

Deps

• Bump axios from 1.13.5 to 1.15.0 by dependabot in #20180
• Bump hono from 4.12.7 to 4.12.12 by dependabot in #20118
• Bump defu from 6.1.4 to 6.1.6 by dependabot in #20104

Other

• (cloudflare) Propagate traceparent to RPC calls - via fetch by JPeer264 in #19991

Bug Fixes 🐛

Deno

• Handle reader.closed rejection from releaseLock() in streaming by andreiborza in #20187
• Avoid inferring invalid span op from Deno tracer by Lms24 in #20128

Other

• (ci) Prevent command injection in ci-metadata workflow by fix-it-felix-sentry in #19899

• (core) Drain buffers in flush() when there is no transport by sbs44 in #20207

• (e2e) Add op check to waitForTransaction in React Router e2e tests by copilot-swe-agent in #20193
• (node-integration-tests) Fix flaky kafkajs test race condition by copilot-swe-agent in #20189

Internal Changes 🔧

Deps

• Bump hono from 4.12.7 to 4.12.12 in /dev-packages/e2e-tests/test-applications/cloudflare-hono by dependabot in #20119
• Bump axios from 1.13.5 to 1.15.0 in /dev-packages/e2e-tests/test-applications/nestjs-basic by dependabot in #20179

Other

• (bugbot) Add rules to flag test-flake-provoking patterns by Lms24 in #20192
• (deps-dev) Bump vite from 7.2.0 to 7.3.2 in /dev-packages/e2e-tests/test-applications/tanstackstart-react by dependabot in #20107
• (react) Remove duplicated test mock by s1gr1d in #20200
• (size-limit) Bump failing size limit scenario by Lms24 in #20186
• Add automatic flaky test detector by nicohrubec in #18684

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[JPeer264]

Thanks a lot for contributing. This fix looks very legitimate - I also took a look on previous PRs moving/adding this hook, it seems that this is something we never considered, but it makes perfect sense this way.