[release-2.11] Fix CVE-2026-42154: validate snappy decoded length in remote read

[katekeiroz-dev]

Summary

• Cherry-pick of 497f8989b from release-2.17 (conflict resolved in codec_test.go)
• Validates snappy decoded length before allocation in the remote read endpoint (/api/v1/read)
• Prevents unauthenticated DoS via crafted snappy payload (snappy bomb)

CVE

• CVE-2026-42154 (CVSS 7.5 HIGH) — Unauthenticated Denial of Service via Snappy Bomb in Prometheus Remote Read Endpoint

Jira

ACM-35419

Test plan

• Cherry-pick applied (minor conflict resolved in test file)
• Includes test TestDecodeReadRequestTooLarge
• CI passes

[openshift-ci]

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[openshift-ci]

@katekeiroz-dev: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/common-test	3411076	link	true	/test common-test

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