fix: combine space-separated terms with & operator in free-text search

[hrodmn]

There is some ambiguity between the free-text STAC API extension and the OGC Records specification (stac-api-extensions/freetext-search#12), so I am not 100% sure this is the right thing to do, but right now if you pass a string with two terms separated by spaces (but not quoted), pgstac will throw a tsquery syntax error (https://github.com/stac-utils/pgstac/actions/runs/18101825325/job/51506541624).

From the OGC Records Spec:

For multiple search terms that are white space separated, only records that contain all the search terms specified, in the order specified and separated by any number of white spaces in one or more of the searched text fields SHALL be in the result set.

This PR replaces spaces with & which I think is the behavior that most closely aligns with the specification but does not account for the order of the terms like the spec suggests. I tried using distance operators (e.g. <10>) but that checks for an exact distance between terms ("distance equals 10") instead of checking for "less than or equal to 10".

[vincentsarago]

re-reading the spec, word1 word2 doesn't mean word1 AND word2. Could we just quote the expression?

[hrodmn]

re-reading the spec, word1 word2 doesn't mean word1 AND word2.

It depends if you are looking at the OGC Records spec or the free-text search STAC API extension spec 😅. I think the collection-search extension means to be based on the OGC Records spec but it is not correctly interpreting that particular case (see stac-api-extensions/freetext-search#12 for discussion).

I think the OGC spec does mean space-separated words like word1 word2 to be word1 AND word2: "For multiple search terms that are white space separated, only records that contain all the search terms specified, in the order specified and separated by any number of white spaces in one or more of the searched text fields SHALL be in the result set."

Could we just quote the expression?

Yeah, that would probably be pretty close to the same thing as using the distance operator I think, but might give users false negatives if there are indeed extra whitespaces between the terms in the text fields.