test(profiling): AgentPhase tag visible on CPU execution samples

[jbachorik]

What does this PR do?:
Adds AgentPhaseProfilingTest to ddprof-test. The test registers llm.agent.phase as a custom attribute slot, sets AWAITING_INFERENCE on a worker thread burning CPU, and asserts that at least one datadog.ExecutionSample JFR event carries the phase tag value by name.

Motivation:
End-to-end verification that the tag encoding flows through writeCurrentContext() and surfaces with the correct field name in JFR. This satisfies the "open verification" item in the spec.

Additional Notes:

• Stacked on: feat(profiling): add AgentPhase enum and ContextSetter.readContextValue #573 (AgentPhase enum).
• Run with ./gradlew :ddprof-test:testRelease --tests "com.datadoghq.profiler.llm.AgentPhaseProfilingTest".
• Note from implementation: JFR field name is the raw attribute name "llm.agent.phase" (no dot→underscore transformation), and AbstractProfilerTest requires overriding getProfilerCommand() to include attributes=llm.agent.phase.

How to test the change?:

./gradlew :ddprof-test:testRelease --tests "com.datadoghq.profiler.llm.AgentPhaseProfilingTest"

For Datadog employees:

• This PR doesn't touch any of that.
• JIRA: PROF-14860

[dd-octo-sts]

CI Test Results

Run: #26822354163 | Commit: 99a8947 | Duration: 14m 32s (longest job)

✅ All 32 test jobs passed

Status Overview

JDK	glibc-aarch64/debug	glibc-amd64/debug	musl-aarch64/debug	musl-amd64/debug
8	-	✅	-	-
8-ibm	-	✅	-	-
8-j9	✅	✅	-	-
8-librca	-	-	✅	✅
8-orcl	-	✅	-	-
11	-	✅	-	-
11-j9	✅	✅	-	-
11-librca	-	-	✅	✅
17	✅	✅	-	-
17-graal	✅	✅	-	-
17-j9	✅	✅	-	-
17-librca	-	-	✅	✅
21	✅	✅	-	-
21-graal	✅	✅	-	-
21-librca	-	-	✅	✅
25	✅	✅	-	-
25-graal	✅	✅	-	-
25-librca	-	-	✅	✅

Legend: ✅ passed | ❌ failed | ⚪ skipped | 🚫 cancelled

Summary: Total: 32 | Passed: 32 | Failed: 0

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Updated: 2026-06-02 13:36:13 UTC