feat: Infix filtering support #2685

[mdhaduk]

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feat: infix filtering support (field:value) #2685

Closes #2685

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Typesense supports exact (field:value) and prefix (field:value*) filtering on string fields, but had no way to filter by substring , users couldn't express "give me all docs where this field contains this value."

This PR adds field:value syntax to filter_by, enabling infix/contains filtering on string and string array fields that have infix: true in their schema.

Changes:

• Detection (filter_result_iterator.cpp): value is now detected as an infix pattern before the existing prefix (value*) check, avoiding misidentification.
• Validation: Returns a 400 error if the field does not have infix: true... the infix index is only built when explicitly enabled.
• Lookup: Reuses the existing Index::search_infix() function (already used for query-time infix search). For multi-token filter values (e.g. Chris P), results
  are intersected (AND). Multiple filter values in a list (e.g. [foo, bar]) are OR'd together.
• Result assembly: Results are stored as a flat filter_result_t (sorted uint32_t[]) -> the same pattern used by numeric and geo filters. Mixed infix + exact/prefix in a single OR list is handled by flattening posting lists and OR'ing with infix results before returning.

Tesing (CollectionFilteringTest.InfixFilterOnTextFields)

• Basic single-token infix on string[] field (ris matches "Chris Evans", "Chris Parnell", "Chris Pine")
• Infix on string field
• No matches case
• OR of multiple infix values ([ris, art])
• Error on field without infix: true (400 response)
• Mixed infix + exact in OR ([ris, Martin])
• Mixed infix + prefix in OR ([ris, Ma*])
• Infix combined with AND on another field (cast: ris && points: >60)

[tharropoulos]

CC: @kishorenc

[tharropoulos]

Is this ready for review?

[mdhaduk]

New contributor, so I just wanted a review of this draft before submitting as PR in case of any obvious pitfalls

Let me know if I should just submit as PR, I think repo does not run clean build on issue of draft PR