Improve http codec injector tests

[PerfectSlayer]

What Does This Do

This PR is a complement to #11640
It brings the same kind of clean up / refactoring to the injector tests.

Motivation

Remove duplicated code, mirror the extractor tests

Additional Notes

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Jira ticket: [PROJ-IDENT]

[bric3]

Thanks for the simplification

[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	14.01 s	14.00 s	[-0.7%; +0.9%] (no difference)
startup:insecure-bank:tracing:Agent	13.00 s	13.04 s	[-1.3%; +0.7%] (no difference)
startup:petclinic:appsec:Agent	16.86 s	16.77 s	[-0.5%; +1.4%] (no difference)
startup:petclinic:iast:Agent	16.88 s	16.95 s	[-1.2%; +0.4%] (no difference)
startup:petclinic:profiling:Agent	16.76 s	16.24 s	[-1.3%; +7.6%] (no difference)
startup:petclinic:sca:Agent	16.92 s	16.70 s	[+0.4%; +2.2%] (maybe worse)
startup:petclinic:tracing:Agent	16.18 s	16.22 s	[-1.1%; +0.7%] (no difference)

Commit:f371fa39 · 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.

[PerfectSlayer]

/merge

[gh-worker-devflow-routing-ef8351]

View all feedbacks in Devflow UI.

2026-06-22 11:31:13 UTC ℹ️ Start processing command /merge

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2026-06-22 11:31:24 UTC ℹ️ MergeQueue: waiting for PR to be ready

This pull request is not mergeable according to GitHub. Common reasons include pending required checks, missing approvals, or merge conflicts — but it could also be blocked by other repository rules or settings.
It will be added to the queue as soon as checks pass and/or get approvals. View in MergeQueue UI.
Note: if you pushed new commits since the last approval, you may need additional approval.
You can remove it from the waiting list with /remove command.

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2026-06-22 11:45:06 UTC ℹ️ MergeQueue: merge request added to the queue

The expected merge time in master is approximately 1h (p90).

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2026-06-22 13:20:39 UTC ℹ️ MergeQueue: This merge request was merged