profiling(ddprof): migrate context bridge to the all-native API (Phase 2)

[rkennke]

What

Migrates the profiler context bridge off the deprecated DirectByteBuffer (DBB) context API onto java-profiler's all-native API (setTraceContext / clearTraceContext / setContextValue / clearContextValue).

This is Phase 2 of the all-native context work (Phase 1 = DataDog/java-profiler#631).

Why

• Fixes the virtual-thread use-after-free: the DBB path cached a DirectByteBuffer over the native OTEP record; on a carrier that a mounted virtual thread later migrated off, the record was freed while the cached buffer kept being written. The all-native API resolves the current carrier's record inside each JNI call — no cached buffer to dangle.
• Per-activation perf: activation collapses from setContext + 2×setContextValue (3 JNI calls) into one setTraceContext (~28% faster on the combined cycle per the design-note benchmark).

Changes

• DatadogProfilingIntegration.activate → one setTraceContext(...) carrying trace/span context + operation & resource attributes; close/clearContext → clearTraceContext().
• DatadogProfiler: setContextValue/clearContextValue/reapplyAppContext/syncNativeAppContext are now all-native. reapplyAppContext uses a per-slot native setContextValue loop — native setContextValue publishes the record (valid=1) even with no active span, so app context stays visible between spans (preserves Restore app context attributes wiped by ddprof setContext on span activation #11646). A native batch reapply is deferred to a measured follow-up (java-profiler PROF-15361).
• AppContextSnapshot simplified to strings-only (the native path resolves each value's encoding via the process-wide cache — no cached id/utf8/snapshotTags DBB read).
• snapshot() uses the new native copyContextTags read (no ThreadContext/DBB, so it observes native writes without resetting the record).
• ContextSetter kept only for offsetOf + size (pure Java); no DBB usage remains.

Dependency / how to build

Requires java-profiler with the all-native API — DataDog/java-profiler#631 (ddprof ≥ the Phase-1 release). Until that ships, build/test against a local snapshot:

# in java-profiler (PR #631 branch):
./gradlew publishToMavenLocal          # com.datadoghq:ddprof:1.47.0-SNAPSHOT
# here:
./gradlew :dd-java-agent:agent-profiling:profiling-ddprof:test -PddprofUseSnapshot=true

Status / verification

• Draft — gated on the java-profiler Phase-1 release and Linux CI verification.
• Compiles cleanly against the local 1.47.0-SNAPSHOT.
• The ddprof test suite (DatadogProfilerTest, …) is Linux-gated (assumeTrue(isLinux())) and was not run on the author's macOS box — needs Linux/CI to exercise behavior.

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