fix: match the ETag response header case-insensitively

[thweetkomputer]

Problem

AsyncHttpManager::HeaderCallback extracted the ETag with a byte-exact "ETag:" prefix compare. HTTP header names are case-insensitive, and HTTP/2 delivers them lowercased (etag:) — curl negotiates HTTP/2 by default over TLS (CURL_HTTP_VERSION_2TLS), so against an h2 endpoint (e.g. GCS) task->etag_ stayed empty.

BootstrapUpsertTermFile then issued the term-file update with an empty ETag, so SetupUploadRequest added neither If-Match nor If-None-Match: the compare-and-swap that fences zombie writers after failover silently degraded to an unconditional PUT, reopening the split-brain window it exists to close (two nodes can both "win" the term-file CAS and write the same partitions).

AWS S3 (HTTP/1.1) and the plain-HTTP MinIO CI config send ETag (capitalized) and were unaffected — which is exactly why existing tests never caught it.

Fix

Match the header name case-insensitively (strncasecmp); the value is untouched. The ETag parsing is extracted into a pure, unit-testable AsyncHttpManager::ParseETagHeader.

Test

object_store_etag unit test feeds the parser header lines directly, so the case handling is exercised offline — no HTTP/2 server needed (the e2e MinIO path can't reproduce this, since MinIO sends ETag). It asserts ETag:, lowercase etag:, and mixed-case Etag:\t… all parse, and that non-ETag / empty / too-short lines are rejected.

[coderabbitai]

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: c86f4724-021a-4d53-a2d5-0384648cc3a3

📥 Commits

Reviewing files that changed from the base of the PR and between 8ba2b33 and 9b3d25f.

📒 Files selected for processing (4)

• include/storage/object_store.h
• src/storage/object_store.cpp
• tests/CMakeLists.txt
• tests/object_store_etag.cpp

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/etag-case-insensitive

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