[performance-profiler] Reuse OTel filestorage Each decoder to reduce allocations

## Hot Path
`libbeat/statestore/backend/otelstorage/store_from_client.go:96-112`, specifically `(*storeFromClient).Each` line 102. The `Walk` callback allocates a new `*jsonValueDecoder` for every stored key, even though the `ValueDecoder` contract says it is invalid after the callback returns (`libbeat/statestore/backend/backend.go:65-70`).

## Profiling Data
**Before:**
```
go test -run=^$ -bench=BenchmarkCRUD/otel_file_storage/Each$ -benchmem -benchtime=1s -count=5 ./libbeat/statestore/backend/benchmarks
BenchmarkCRUD/otel_file_storage/Each: median 683078 ns/op, 485233 B/op, 20021 allocs/op
```
A reproduced five-run baseline on the current checkout measured 664225-667674 ns/op with the same 485233-485234 B/op and 20021 allocs/op. The allocation profile attributed 576 MB (34.99% flat) to `storeFromClient.Each.func1`.

## Proposed Change
Create one `jsonValueDecoder` before calling `walker.Walk`, assign its `raw` field to the current `value` inside the callback, and pass that decoder to `fn`. This is behavior-preserving because the backend contract explicitly invalidates the decoder after each callback.

## Results
**After (same benchmark command):**
```
BenchmarkCRUD/otel_file_storage/Each: median 578239 ns/op, 245265 B/op, 10022 allocs/op
```
**Improvement:** 15.35% lower ns/op, 49.45% fewer bytes/op, and 49.94% fewer allocations/op.

## Verification
- `go test ./libbeat/statestore/backend/otelstorage` passed, including `TestStoreFromClient_Each`.
- The temporary optimization was reverted after measurement; no repository changes remain.
- The decoder remains valid for all repeated `Decode` calls within each callback and is intentionally invalidated when the callback returns.

## Evidence
- Benchmark: `libbeat/statestore/backend/benchmarks/benchmarks_test.go:137-149` preloads 10,000 keys and repeatedly calls `Each`.
- Implementation: `libbeat/statestore/backend/otelstorage/store_from_client.go:101-104` allocates the decoder per key.
- Contract: `libbeat/statestore/backend/backend.go:65-70`.
- Searches found no issue or pull request specifically tracking `storeFromClient`/`jsonValueDecoder` reuse. Existing OTel filestorage benchmark work in PR #50130 documents `Each` overhead but does not address this allocation.

Suggested action: add a focused regression benchmark or retain the existing benchmark while reusing the decoder instance in `Each`.

---
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Start in libbeat/statestore/backend/otelstorage/store_from_client.go:101-104 and read the ValueDecoder contract in libbeat/statestore/backend/backend.go:65-70. Run BenchmarkCRUD/otel_file_storage/Each from libbeat/statestore/backend/benchmarks, then run go test ./libbeat/statestore/backend/otelstorage. Done means Each preserves callback decoding while reducing its measured allocations and passing the existing tests.

Written by the indexing model from the issue text.