fix: serve stale read data on backend errors

[szibis]

Summary

• stop masking backend failures for detected fields, detected labels, detected field values, volume, and volume range as empty successful responses
• serve stale last-good local cache data for these read surfaces when a backend refresh fails, and otherwise return a real upstream-style error
• honor non-success /select/logsql/hits responses in the volume helpers and add regressions for Sand-shaped no-data-after-refresh failures

Validation

• go test ./internal/cache
• go test ./internal/proxy -run "Test(Drilldown_Detected(Fields|Labels|FieldValues)_|Contract_Volume(ResponseFormat|Range_ResponseFormat|_BypassesNearNowStaleCache|_ServesStaleCacheWhenNearNowRefreshFails|_ReturnsGatewayErrorWithoutCacheOnBackendFailure))"
• go test ./internal/proxy

[github-actions]

PR Quality Report

Compared against base branch main.

Coverage and tests

Signal	Base	PR	Delta
Test count	1787	1794	7
Coverage	89.9%	89.9%	0.0% (stable)

Compatibility

Track	Base	PR	Delta
Loki API	100.0%	11/11 (100.0%)	0.0% (stable)
Logs Drilldown	100.0%	17/17 (100.0%)	0.0% (stable)
VictoriaLogs	100.0%	11/11 (100.0%)	0.0% (stable)

Compatibility components

Track	Component	Base	PR	Delta
Loki API	label_values	2/2 (100.0%)	2/2 (100.0%)	0.0% (stable)
Loki API	labels	4/4 (100.0%)	4/4 (100.0%)	0.0% (stable)
Loki API	metrics	2/2 (100.0%)	2/2 (100.0%)	0.0% (stable)
Loki API	otel	1/1 (100.0%)	1/1 (100.0%)	0.0% (stable)
Loki API	query_range	1/1 (100.0%)	1/1 (100.0%)	0.0% (stable)
Loki API	series	1/1 (100.0%)	1/1 (100.0%)	0.0% (stable)
Logs Drilldown	detected_fields	11/11 (100.0%)	11/11 (100.0%)	0.0% (stable)
Logs Drilldown	label_values	1/1 (100.0%)	1/1 (100.0%)	0.0% (stable)
Logs Drilldown	level_volume	2/2 (100.0%)	2/2 (100.0%)	0.0% (stable)
Logs Drilldown	patterns	1/1 (100.0%)	1/1 (100.0%)	0.0% (stable)
Logs Drilldown	service_logs	1/1 (100.0%)	1/1 (100.0%)	0.0% (stable)
Logs Drilldown	service_selection	1/1 (100.0%)	1/1 (100.0%)	0.0% (stable)
VictoriaLogs	detected_fields	4/4 (100.0%)	4/4 (100.0%)	0.0% (stable)
VictoriaLogs	field_values	3/3 (100.0%)	3/3 (100.0%)	0.0% (stable)
VictoriaLogs	index_stats	1/1 (100.0%)	1/1 (100.0%)	0.0% (stable)
VictoriaLogs	stream_translation	1/1 (100.0%)	1/1 (100.0%)	0.0% (stable)
VictoriaLogs	synthetic_labels	1/1 (100.0%)	1/1 (100.0%)	0.0% (stable)
VictoriaLogs	volume_range	1/1 (100.0%)	1/1 (100.0%)	0.0% (stable)

Performance smoke

Lower CPU cost (ns/op) is better. Lower benchmark memory cost (B/op, allocs/op) is better. Higher throughput is better. Lower load-test memory growth is better. Benchmark rows are medians from repeated samples.

Signal	Base	PR	Delta
QueryRange cache-hit CPU cost	1348.0 ns/op	1358.0 ns/op	+0.7% (stable)
QueryRange cache-hit memory	200.0 B/op	200.0 B/op	0.0% (stable)
QueryRange cache-hit allocations	7.0 allocs/op	7.0 allocs/op	0.0% (stable)
QueryRange cache-bypass CPU cost	1669.0 ns/op	1678.0 ns/op	+0.5% (stable)
QueryRange cache-bypass memory	272.0 B/op	275.0 B/op	+1.1% (stable)
QueryRange cache-bypass allocations	7.0 allocs/op	7.0 allocs/op	0.0% (stable)
Labels cache-hit CPU cost	678.4 ns/op	673.1 ns/op	-0.8% (stable)
Labels cache-hit memory	56.0 B/op	56.0 B/op	0.0% (stable)
Labels cache-hit allocations	4.0 allocs/op	4.0 allocs/op	0.0% (stable)
Labels cache-bypass CPU cost	842.9 ns/op	832.4 ns/op	-1.2% (stable)
Labels cache-bypass memory	84.0 B/op	84.0 B/op	0.0% (stable)
Labels cache-bypass allocations	4.0 allocs/op	4.0 allocs/op	0.0% (stable)
High-concurrency throughput	102019.0 req/s	107535.0 req/s	+5.4% (stable)
High-concurrency memory growth	0.8 MB	0.8 MB	0.0% (stable)

State

• Coverage, compatibility, and sampled performance are reported here from the same PR workflow.
• This is a delta report, not a release gate by itself. Required checks still decide merge safety.
• Performance is a smoke comparison, not a full benchmark lab run.
• Delta states use the same noise guards as the quality gate (percent + absolute + low-baseline checks), so report labels match merge-gate behavior.