[ES|QL] Allow single fork branch

[pmpailis]

Removing the requirement for > 2 fork branches. So a query like

            FROM test
            | WHERE id > 2
            | FORK
               ( WHERE content:"fox" )
            """

will now be allowed, and only fork1 will be added to the result set.

Closes #135825

[elasticsearchmachine]

Hi @pmpailis, I've created a changelog YAML for you.

[pmpailis]

@ioanatia went through the documentation but could not find any specific references to the >= 2 branches limitation that needed to be updated.

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[ioanatia]

can we get a test for FORK with no branches? I think the grammar does not support it, but I don't think I added a test for that.

[pmpailis]

Updated in a8ce7b9. I've also added a test in FuseIT to consume the result of a single-branched FORK. Let me know if you see any issues :)

[ioanatia]

also thought a bit more about the discussion we had around doing an optimization, like in the LogicalPlanBuilder when a single FORK branch is being used, to not even use the FORK logical plan, but have a "linear" plan.
You had a good intuition that we do have certain limitations for FORK, for example we only allow a single FORK command per query. If we do this optimization, we break this limitation which could be surprising for users.
So just removing the validation for now, with no optimizations, makes sense to me 👍

[pmpailis]

Thanks for reviewing this (again) @ioanatia !