[watson]

Motivation

DEBUG-6251: follow-up to #5107, which only created RUM custom actions for probes evaluated at function entry. EXIT probes were skipped because their condition is only known at exit, and the public RUM API can't backdate the start of an action. For now, EXIT probes get a point-in-time action at the function exit, so we can still find the RUM sessions in which they fired.

Changes

The first qualifying hit of an EXIT probe (one that passes sampling and its exit condition) now calls DD_RUM.addAction() at function exit:

 onEntry
   ENTRY probe → startAction(name, { actionKey: snapshotId, context: { probe, snapshot } })
 onReturn / onThrow
   ENTRY probe → stopAction(name, { actionKey: snapshotId, context: { outcome, error } })
+  EXIT probe, after the exit condition passes
+    → addAction(name, { debugger: { probe, snapshot, outcome, error } })
   queue snapshot (reuses the action's snapshot id)

• Same action name (probe: <method> (<file>)) and the same final context as ENTRY probe actions, and the snapshot sent for that hit reuses the action's snapshot id.
• Same rules as ENTRY probes: first qualifying hit per probe instance, the probe stays armed until DD_RUM is on the page, trackProbeHitsAsRumActions: false disables it, and RUM API failures can't break the instrumented function or prevent the snapshot.
• Condition-evaluation errors and capture-expression timeouts (no snapshot sent) don't create an action.
• README and trackProbeHitsAsRumActions JSDoc updated.

Worth a reviewer's eye (AI-assisted implementation):

• No duration: addAction events have no action.loading_time at all, rather than loading_time: 0. Calling startAction/stopAction back to back would instead record a meaningless sub-millisecond duration.
• Timestamp: the action's date is when addAction runs, after the exit condition, message and capture expressions are evaluated (bounded by the existing 10ms capture deadline). The snapshot's timestamp is taken just before the condition, so the two can differ slightly.
• Telemetry: like startAction/stopAction in ✨ [DEBUG-6251] Start a RUM action on the first hit of each browser probe #5107, the public addAction records add-action usage telemetry. Avoiding that, and supporting real EXIT-probe durations, still needs an internal RUM API designed with the RUM team.

Test instructions

1. yarn test:unit --spec 'packages/browser-debugger/src/**/*.spec.ts'. The old should not start an action for EXIT probes test is replaced by a RUM actions > for EXIT probes suite in api.spec.ts.
2. yarn test:e2e test/e2e/scenario/debugger.scenario.ts -g "RUM action on the first hit". The new add a point-in-time RUM action on the first hit of an EXIT probe scenario calls an instrumented function twice with an EXIT probe, then checks that exactly one custom action is sent, with no loading_time, the matching snapshot id and outcome: 'return'.
3. Manually, in an app with RUM + the debugger and the build plugin: set an EXIT log probe, trigger the function, and check the RUM explorer for @action.type:custom @context.debugger.probe.id:<probe id>.

Locally: 883/883 browser-debugger unit tests pass, and both RUM action E2E scenarios (ENTRY and EXIT) pass on Chromium.

Checklist

• Tested locally
• Tested on staging
• Added unit tests for this change.
• Added e2e/integration tests for this change.
• Updated documentation and/or relevant AGENTS.md file

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