🍒 11949 - Fix RUM injection for offset response writes

[PerfectSlayer]

Backport #11949 to release/v1.64.x

[datadog-datadog-us1-prod]

More details

The offset-write fix preserves response content and injects exactly once across sliced bulk writes, marker-at-end inputs, and markers split across write boundaries. Direct execution of both byte and character implementations matched a reference injector across 20,000 randomized scenarios; the Gradle test task itself could not start because the wrapper distribution download was unavailable.

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_{📊 Validated against 20000 scenarios · Open Bits AI session}

_{🤖 Datadog Autotest · Commit bed0fd5 · What is Autotest? · @DataDog review to ask questions · 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	13.86 s	[+0.3%; +1.6%] (maybe worse)
startup:insecure-bank:tracing:Agent	12.97 s	13.00 s	[-1.0%; +0.6%] (no difference)
startup:petclinic:appsec:Agent	17.00 s	16.91 s	[-0.3%; +1.4%] (no difference)
startup:petclinic:iast:Agent	16.94 s	16.98 s	[-1.0%; +0.5%] (no difference)
startup:petclinic:profiling:Agent	16.70 s	16.81 s	[-1.8%; +0.5%] (no difference)
startup:petclinic:sca:Agent	16.95 s	16.81 s	[-0.1%; +1.8%] (no difference)
startup:petclinic:tracing:Agent	15.66 s	16.15 s	[-7.2%; +1.1%] (no difference)

Commit: bed0fd59 · CI Pipeline · Benchmarking Platform UI

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