test(e2e): assert msgpack decode-error controls

[houseme]

Related Issues

Part of rustfs/backlog#1435 and rustfs/backlog#1431.

Summary of Changes

This PR extends the inline fast-path mixed-msgpack E2E OTLP collector to capture rustfs_system_network_internode_msgpack_json_decode_error_total with direction, message, and codec labels alongside the existing msgpack/json fallback counter. The mixed compatibility tests now assert that decode-error counters stay unchanged for the same fallback-control series already covered by the release-window scenarios.

The covered black-box paths are multipart, partNumber reads, SSE-S3, compression, and cold-tier transition fallback controls. The implementation is test-only and does not change the public API, wire protocol, storage format, runtime configuration, or production metric emission.

Verification

• cargo fmt --all --check
• git diff --check
• cargo clippy -p e2e_test --lib -- -D warnings
• cargo test -p e2e_test --lib inline_fast_path_cluster_test::records_msgpack_decode_error_metric_with_codec_label -- --exact --nocapture
• cargo test -p e2e_test --lib inline_fast_path_cluster_test::four_node_mixed_msgpack_compat_mode_preserves_fallback_controls -- --exact --nocapture
• cargo test -p e2e_test --lib inline_fast_path_cluster_test::four_node_mixed_msgpack_compat_mode_preserves_fallback_controls_during_transition -- --exact --nocapture

Impact

No user-facing behavior change. This only strengthens E2E coverage for msgpack/json rollout observability by making decode-error label dimensions visible to the test collector and by requiring no decode-error counter delta during the mixed-version fallback-control scenarios.

Additional Notes

Adversarial review: correctness attacked missing codec label capture and stale counter snapshots, so the PR includes a direct collector unit test for direction/message/codec; simplicity attacked duplicated OTLP parsing, so msgpack counter collection is shared through one helper; coverage attacked false-positive zero deltas, so the E2E assertions are paired with the collector label unit test.

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