fix: use bare exists json tag on v3 FilterString, not $exists (#4678)

Author: gergely-kurucz-konghq

.agents/skills/api-filters/SKILL.md

@@ -91,7 +91,7 @@ Operator constants live in `api/v3/filters/parse.go` as `OpEq`, `OpNeq`, `OpGt`,
 | `FilterLabel`       | `Eq`, `Neq`, `Contains`, `Oeq`, `Ocontains` (label map value predicates)       |
 | `FilterLabels`      | type alias: `map[string]FilterLabel`                                           |
 
-The `Exists` field is serialized under the JSON key `$exists` so it does not collide with a literal `exists` operator in flattened encodings.
+The wire operator for `Exists` is plain `exists` (see `OpExists` in `api/v3/filters/parse.go`), matching its `json:"exists,omitempty"` tag — don't confuse it with the unrelated `$`-prefixed Mongo-style tags used by the v1 API (`api/api.gen.go`).
 
 **Important:** the API-layer types do NOT have `Validate()` methods. Validation (mutual exclusivity, complexity bounds, format checks) happens on the internal `pkg/filter.*` predicates — typically from the service input struct's own `Validate()`, calling `f.Validate()` on each non-nil filter.
 

.agents/skills/api/rules/aip-160-filtering.md

@@ -48,11 +48,13 @@ The authoritative operator surface for each Common type is defined in `api/spec/
 
 The Go types in `api/v3/filters/filter.go` deliberately expose a **superset** of operators beyond what the matching OAS `Common.*FieldFilter` advertises, so a single Go struct can back several endpoints with different narrow OAS surfaces:
 
-- `filters.FilterString` adds `gt`/`gte`/`lt`/`lte` and an `$exists` check on top of `Common.StringFieldFilter`'s operators
+- `filters.FilterString` adds `gt`/`gte`/`lt`/`lte` and an `exists` check on top of `Common.StringFieldFilter`'s operators
 - `filters.FilterNumeric` adds `neq` and `oeq`
 - `filters.FilterDateTime` matches its Common type exactly (`eq`/`gt`/`gte`/`lt`/`lte`)
 
-The existence check (`$exists`) and `nexists` are AIP-160 operators, but they are not currently modeled in any `Common.*FieldFilter` `oneOf` shape, so picking e.g. `Common.StringFieldFilter` in TypeSpec does not advertise them. Do not rely on `gt`/`gte`/`lt`/`lte` on string fields as a stable contract — the Common type is the public surface.
+The existence check (`exists`) and `nexists` are AIP-160 operators, but they are not currently modeled in any `Common.*FieldFilter` `oneOf` shape, so picking e.g. `Common.StringFieldFilter` in TypeSpec does not advertise them. Do not rely on `gt`/`gte`/`lt`/`lte` on string fields as a stable contract — the Common type is the public surface.
+
+Note: the v3 `exists` operator is unrelated to the `$`-prefixed Mongo-style tags (`$eq`, `$in`, ...) used by the v1 API (`api/api.gen.go`) — don't conflate the two.
 
 ## Operators
 

api/v3/filters/filter.go

@@ -80,7 +80,7 @@ type FilterString struct {
 	Ocontains []string `json:"ocontains,omitempty"`
 
 	// Exists requires the field to be present (true) or absent (false).
-	Exists *bool `json:"$exists,omitempty"`
+	Exists *bool `json:"exists,omitempty"`
 }
 
 // FilterULID represents a filter operation on a string field that satisfies ULID format.