feat(profiling): register llm.agent.phase slot and add phase set/clear API

[jbachorik]

What Does This Do

Registers llm.agent.phase as a context attribute slot in DatadogProfiler at startup (when isLlmPhaseAttributeEnabled is true) and exposes setAgentPhase(String phaseToken) / clearAgentPhase() for use by instrumentation modules.

Motivation

ContextSetter is built once; slots not present at construction time have offset -1 (no-op). This change wires the registration into the existing orderedContextAttributes pipeline, consuming 1 of the 10 available slots. The named API (setAgentPhase/clearAgentPhase) hides the raw offset from instrumentation callers.

Part of PROF-14858. Stacked on #11528 (config flag). Next: [ddt-3-lc4j] (LangChain4j hooks).

Additional Notes

• llm.agent.phase appears in the profiler start command's attributes= parameter when the feature flag is on.
• No change to any sampler hot path; slot lookup and registration happen only at profiler startup.

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Jira ticket: PROF-14862

[datadog-official]

✨ Fix all issues with BitsAI

⚠️ Warnings

🚦 10 Pipeline jobs failed

DataDog/apm-reliability/dd-trace-java | spotless     

🔧 Fix in code (Fix with Cursor).

Spotless check failed due to format violations in src/main/java/com/datadog/profiling/ddprof/DatadogProfiler.java. Unexpected import changes detected.

Run system tests | main / End-to-end #2 / akka-http 2     

🔄 Retry job. This looks flaky and may succeed on retry.

Connection to Docker registry failed: request canceled while waiting for connection. Fetch failed after 4 attempts.

Run system tests | main / End-to-end #5 / spring-boot 5     

🔄 Retry job. This looks flaky and may succeed on retry.

1 failed test. Assertion Error: Request failed: None. Expected status code 200 but got None instead at tests/ffe/test_dynamic_evaluation.py:237.

View all 10 failed jobs.

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