Added unnest

[ilongin]

Adds DataChain.unnest(col) - expands a list column into rows, one row per element. Sibling columns are repeated. Equivalent to SQL UNNEST, Spark explode, pandas .explode(). Empty lists and None drop the source row (inner semantics).

Naming

The industry-standard name for this is explode, but datachain already has an explode() that does something different (JSON-string-to-columns). To get to the standard name without breaking users, the plan is:

1. Ship unnest now (this PR).
2. In a follow-up, rename the existing explode to something descriptive (e.g. expand_json) and keep explode as a deprecated alias that warns.
3. In a later release, once explode is no longer the JSON helper, alias unnest as explode and deprecate unnest.
4. After a deprecation window, drop the old names.

This avoids a single confusing rename and lets users migrate one step at a time.

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[codecov]

Codecov Report

❌ Patch coverage is 93.54839% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/datachain/lib/dc/datachain.py	93.54%	1 Missing and 1 partial ⚠️

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[dmpetrov]

@ilongin it might be a little bit premature implementing this. I expect a lot of unset-challenges with the coming Union[] #1824 that's required for proper LLM resposes decoding.

I'd wait for Union first.

Also, as we discussed before, it’s better not to pick up issues that haven’t been prioritized yet - or ask about it (I'm not sure if you had some other discussions about this issue).