Add test agent session token support

[PerfectSlayer]

What Does This Do

This PR introduces an optional test agent session token for both trace and telemetry writers.

Motivation

The goal is to be able to reuse the existing test agent running in CI but having isolated session.
This will save running multiple test agents to avoid test pollution.

Additional Notes

This PR is part of improving smoke test capabilities.

Contributor Checklist

• Format the title according to the contribution guidelines
• Assign the type: and (comp: or inst:) labels in addition to any other useful labels
• Avoid using close, fix, or any linking keywords when referencing an issue
  Use solves instead, and assign the PR milestone to the issue
• Update the CODEOWNERS file on source file addition, migration, or deletion
• Update public documentation with any new configuration flags or behaviors
• Once approved, use merge queue to merge the PR

Jira ticket: APMLP-1247

[datadog-datadog-prod-us1]

Bits has a CI fix ready

Warning

This comment is related to an earlier commit.

🟢 Investigated · 🟢 Fix prepared · ⚪ Validation skipped · 🟠 Ready

Added the formatter-required blank lines around X_DATADOG_TEST_SESSION_TOKEN in dd-trace-core/src/main/java/datadog/trace/common/writer/ddagent/DDAgentApi.java.

Commit fix to this PR

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View in Datadog | Reviewed commit a9bceec · Any feedback? Reach out in #deveng-pr-agent

[datadog-datadog-prod-us1]

More details

The new session token is consistently read from DD_TEST_AGENT_SESSION_TOKEN and attached to trace-agent and agent-telemetry requests, while API-key-authenticated intake keeps its existing authentication path. No clear diff-only behavioral regression was identified; focused execution was blocked by the unavailable Gradle download and Java 25 toolchain.

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_{🤖 Datadog Autotest · Commit 6d68087 · What is Autotest? · Any feedback? Reach out in #autotest}

[dd-octo-sts]

🟢 Java Benchmark SLOs — All performance SLOs passed

Suite	Status
Startup	🟢 pass

SLO thresholds are defined here based on automatically generated metrics. A warning is raised when results are within 5% of the threshold.

PR vs. master results

Scenario	Candidate	master	Δ (95% CI of mean)
startup:insecure-bank:iast:Agent	13.99 s	13.91 s	[-0.2%; +1.3%] (no difference)
startup:insecure-bank:tracing:Agent	12.94 s	13.06 s	[-2.0%; +0.0%] (no difference)
startup:petclinic:appsec:Agent	16.98 s	16.82 s	[-0.1%; +2.0%] (no difference)
startup:petclinic:iast:Agent	16.48 s	16.90 s	[-6.8%; +1.7%] (no difference)
startup:petclinic:profiling:Agent	16.71 s	16.89 s	[-2.3%; +0.2%] (no difference)
startup:petclinic:sca:Agent	16.45 s	16.99 s	[-7.5%; +1.2%] (no difference)
startup:petclinic:tracing:Agent	16.08 s	16.25 s	[-2.0%; -0.2%] (maybe better)

Commit: 775ec981 · CI Pipeline · Benchmarking Platform UI

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Load and DaCapo benchmarks can be triggered manually in the GitLab pipeline. Results will appear in the Benchmarking Platform UI after completion.

[chatgpt-codex-connector]

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Reviewed commit: 6d68087e16

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[chatgpt-codex-connector]

Use the session token when checking agent failures

After moving smoke jobs onto the test-agent template, validation still runs .gitlab/check_test_agent_results.sh, whose /test/trace_check/summary and /test/trace_check/failures calls do not pass DD_TEST_AGENT_SESSION_TOKEN. In the shared-agent scenario this feature is meant to support, those endpoints report all sessions when no token is supplied, so an unrelated session's trace-check failure can still fail this job despite the writer tagging its trace requests.

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