fix: clone label values to prevent buffer reuse-after-free

[simonswine]

Reader.LabelValues (no-matcher path) returns yoloString aliases into the
pooled TSDB buffer. Once the dataset closes, that buffer is returned to
bufferpool and can be immediately reused by a concurrent request — overwriting
the strings while they are still held by the aggregator.

Fix by cloning each value with strings.Clone before the dataset is closed.

Adds Test_LabelValues_NoMatcher_BufferReuse: 20 goroutines × 20 iterations
of a no-matcher LabelValues query. Without the fix, concurrent buffer
recycling corrupts the returned strings (e.g. "test-app" becomes "codeInt64").
Run with -race to also trigger the detector on the underlying read/write race.

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Note

Medium Risk
Touches a hot query path and changes memory/lifetime behavior to eliminate a potential data race from pooled buffer reuse; correctness improves but adds per-value allocations that could impact performance under high cardinality.

Overview
Fixes the no-matcher LabelValues fast path to clone returned strings (strings.Clone) so label values no longer alias a pooled TSDB/index buffer that may be recycled before the report is marshaled.

Adds a concurrent regression test (Test_LabelValues_NoMatcher_BufferReuse) that issues many parallel label-values queries and asserts stable results (and triggers -race without the fix) to guard against buffer reuse corruption/data races.

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