fix(joiner): add bounds checking on leaf reads

[janos]

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• I have read the coding guide.
• My change requires a documentation update, and I have done it.
• I have added tests to cover my changes.
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Description

This PR fixes a panic (crash) and an infinite loop (hang/timeout) when reading malformed or corrupted trees in the joiner.

When the joiner processes a malformed tree where intermediate or root chunks advertise spans larger than the actual data present in their children it hangs in io.Copy as it always returns (0, nil), or panics when the data is requested by the look-ahead reader langos as the io.SeekEnd on joiner's Seek() reports longer data.

Tests are added for fix verification. The real world chunks and initial testing was provided by @acud, thanks.

Open API Spec Version Changes (if applicable)

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• This PR contains code that has been generated by an LLM.
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