Send Java SDK identity with Feature Flagging EVP events

[leoromanovsky]

Motivation

Attribute Feature Flagging exposures and flag evaluations to the Java SDK on both local EVP proxy and direct-intake routes. Initial direct delivery already shipped; this PR isolates producer identity from the transport hardening in #12477. Tracks FFL-3016 within the FFLSDK-187 rollout.

Changes

• Send DD-EVP-ORIGIN: dd-trace-java and DD-EVP-ORIGIN-VERSION using the tracer version.
• Add immutable per-request header configuration to the shared backend factory.
• Verify local/direct identity headers while retaining credential-isolation and one-send assertions.

Decisions

• Stack on leoromanovsky/fflsdk-187-java-evp-fallback, not another language or unrelated SDK branch.
• Forwarding-capability requirements, routing, retries, credentials and lifecycle remain in Harden Agentless Feature Flags EVP delivery #12477. Required header names are independent of the identity values emitted here.
• No new configuration or system-test changes.

Validation

• Exact base: 1b5766b56c67bb7151a084abb752e62e5bb4b332.
• Exact candidate: 96a3427bb9bcb640165306d16af141e9455e2cc4.
• ./gradlew :communication:spotlessApply :products:feature-flagging:feature-flagging-lib:spotlessApply :communication:test :products:feature-flagging:feature-flagging-lib:test --console=plain --max-workers=4: PASS; communication 235/235, feature-flagging-lib 279/279.
• git diff --check: PASS. New commit is GitHub verified: verified=true, reason=valid.
• No current-candidate system-tests, dogfooding or backend-intake run. Older artifact results are not claimed for this split.
• Fresh CI remains separate from local validation. This PR remains a draft.

[linear-code]

FFLSDK-187

[datadog-datadog-prod-us1]

🎯 Code Coverage (details)
• Patch Coverage: 20.00%
• Overall Coverage: 59.09% (-0.04%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 96a3427 | Docs | Give us feedback!}

[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	13.95 s	13.95 s	[-0.8%; +0.9%] (no difference)
startup:insecure-bank:tracing:Agent	12.83 s	12.98 s	[-1.9%; -0.4%] (maybe better)
startup:petclinic:appsec:Agent	17.62 s	17.30 s	[+0.8%; +2.9%] (maybe worse)
startup:petclinic:iast:Agent	17.36 s	17.56 s	[-1.9%; -0.4%] (maybe better)
startup:petclinic:profiling:Agent	17.28 s	17.38 s	[-1.9%; +0.7%] (no difference)
startup:petclinic:sca:Agent	17.56 s	17.40 s	[-0.1%; +2.0%] (no difference)
startup:petclinic:tracing:Agent	16.51 s	16.65 s	[-1.9%; +0.2%] (no difference)

Commit: 96a3427b · CI Pipeline · Benchmarking Platform UI

---

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