fix: correctly handle non-existing ids

[dkharms]

Description

Rarely we encountered panics (out of bounds access) and I decided to shove the stacktrace into Fable and observe how it will handle the bug. Well, it found the root-cause.

{"level":"error","ts":"2026-07-07T14:13:34.054Z","message":"runtime error: index out of range [1508] with length 1508"}
goroutine 13507559 [running]:
runtime/debug.Stack()
    runtime/debug/stack.go:26 +0x5e
runtime/debug.PrintStack()
    runtime/debug/stack.go:18 +0x13
github.com/ozontech/seq-db/util.Recover({0x192a3a0?, 0xc00063af00?}, {0x729e409aa098, 0xc1e0afbae8})
    github.com/ozontech/seq-db/util/err.go:27 +0x65
github.com/ozontech/seq-db/util.RecoverToError({0x1634bc0?, 0xc1e0afbae8?}, {0x192a3a0, 0xc00063af00})
    github.com/ozontech/seq-db/util/err.go:33 +0x4f
github.com/ozontech/seq-db/fracmanager.fracFetch.func1()
    github.com/ozontech/seq-db/fracmanager/fetcher.go:110 +0x4d
panic({0x1634bc0?, 0xc1e0afbae8?})
    runtime/panic.go:783 +0x132
github.com/ozontech/seq-db/frac/sealed/seqids.(*unpackCache).GetValByLID(...)
    github.com/ozontech/seq-db/frac/sealed/seqids/unpack_cache.go:47
github.com/ozontech/seq-db/frac/sealed/seqids.(*Provider).MID(0xc100e98550, 0xf15e4)
    github.com/ozontech/seq-db/frac/sealed/seqids/provider.go:54 +0x71
github.com/ozontech/seq-db/frac.(*sealedIDsIndex).GetMID(0xc0f764d0a0, 0xf15e4)
    github.com/ozontech/seq-db/frac/sealed_index.go:148 +0x35
github.com/ozontech/seq-db/frac.(*sealedFetchIndex).findLIDs(0xc100e985f0, {0xc1dad45630, 0x1, 0xc016b9d500?})
    github.com/ozontech/seq-db/frac/sealed_index.go:360 +0x13b
github.com/ozontech/seq-db/frac.(*sealedFetchIndex).GetDocPos(0xc100e985f0, {0xc1dad45630?, 0xc?, 0x1?}, 0x0)
    github.com/ozontech/seq-db/frac/sealed_index.go:302 +0x26
github.com/ozontech/seq-db/frac/processor.IndexFetch({0xc1dad45630, 0x1, 0x1}, 0x0, 0xc0f764d080, {0x19242a0, 0xc100e985f0}, {0xc58016d668, 0x1, 0x1})
    github.com/ozontech/seq-db/frac/processor/fetch.go:16 +0xa6
github.com/ozontech/seq-db/frac.(*sealedDataProvider).Fetch(0xc486c761b0, {0xc1dad45630, 0x1, 0x1}, 0x0)
    github.com/ozontech/seq-db/frac/sealed_index.go:105 +0x2b2
github.com/ozontech/seq-db/frac.(*Sealed).Fetch(0x108e57c?, {0x1925690?, 0xc0b6564230?}, {0xc1dad45630, 0x1, 0x1}, 0x0)
    github.com/ozontech/seq-db/frac/sealed.go:477 +0xad
github.com/ozontech/seq-db/fracmanager.fracFetch({0x1925690?, 0xc0b6564230?}, {0x192a088?, 0xc3533a2c30?}, {0xc1dad45630?, 0xc140d549a0?, 0x3e8?}, 0x70?)
    github.com/ozontech/seq-db/fracmanager/fetcher.go:114 +0xb6
github.com/ozontech/seq-db/fracmanager.(*Fetcher).fetchDocsAsync.func2()
    github.com/ozontech/seq-db/fracmanager/fetcher.go:87 +0xb8
created by github.com/ozontech/seq-db/fracmanager.(*Fetcher).fetchDocsAsync in goroutine 13507558
    github.com/ozontech/seq-db/fracmanager/fetcher.go:85 +0x1da
{"level":"error","ts":"2026-07-07T14:13:34.055Z","message":"fetch error","error":"internal error: fetch panicked on fraction seq-db-01KWYD9DBNAYK3Y859ZGRCC8A4, error=runtime error: index out of range [1508] with length 1508"}

---

• I have read and followed all requirements in CONTRIBUTING.md;
• I used LLM/AI assistance to make this pull request;

If you have used LLM/AI assistance please provide model name and full prompt:

Model: Fable 5
Prompt: Asked to debug the panic

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.22%. Comparing base (886c9bd) to head (1af2d99).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #458      +/-   ##
==========================================
- Coverage   70.38%   70.22%   -0.17%     
==========================================
  Files         232      232              
  Lines       18343    18343              
==========================================
- Hits        12911    12881      -30     
- Misses       4466     4486      +20     
- Partials      966      976      +10     

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