feat(logsql): support the json_array_concat pipe (VL v1.51+)

[szibis]

What

Adds internal/logsql support for VictoriaLogs' json_array_concat pipe:

| json_array_concat [delimiter] [from <src_field>] [as <result_field>]

PipeJSONArrayConcat AST node + parser + round-trip String(), mirroring the existing PipeSplit (all parts optional).

Why

VictoriaLogs added this pipe upstream (v1.51), which made the Check VL upstream AST coverage job start failing repo-wide (new construct not in scripts/vl-ast-coverage.json). This closes that gap and lets the proxy parse/round-trip user LogsQL that uses the pipe.

Verification

• go build ./... ✅, go vet ✅, go test ./internal/logsql/ ✅
• FuzzParseRoundTrip 6M execs, no crash ✅
• scripts/check-vl-ast-coverage.py → "No gaps found" ✅ (was reporting json_array_concat as a new construct)

First of the VictoriaLogs v1.52 native-feature series.

[github-actions]

PR Quality Report

Compared against base branch main.

Coverage and tests

Signal	Base	PR	Delta
Test count	4795	4801	6
Coverage	85.0%	85.1%	+0.1% (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)

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	2970.0 ns/op	2944.0 ns/op	-0.9% (stable)
QueryRange cache-hit memory	1184.0 B/op	1184.0 B/op	0.0% (stable)
QueryRange cache-hit allocations	29.0 allocs/op	29.0 allocs/op	0.0% (stable)
QueryRange cache-bypass CPU cost	3186.0 ns/op	3176.0 ns/op	-0.3% (stable)
QueryRange cache-bypass memory	1245.0 B/op	1245.0 B/op	0.0% (stable)
QueryRange cache-bypass allocations	30.0 allocs/op	30.0 allocs/op	0.0% (stable)
Labels cache-hit CPU cost	703.3 ns/op	674.8 ns/op	-4.1% (stable)
Labels cache-hit memory	48.0 B/op	48.0 B/op	0.0% (stable)
Labels cache-hit allocations	3.0 allocs/op	3.0 allocs/op	0.0% (stable)
Labels cache-bypass CPU cost	1275.0 ns/op	1247.0 ns/op	-2.2% (stable)
Labels cache-bypass memory	391.0 B/op	391.0 B/op	0.0% (stable)
Labels cache-bypass allocations	14.0 allocs/op	14.0 allocs/op	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.