feat(#3996): resolve image output capability from models.dev

Resolve an omitted output_capabilities.image flag from the model's models.dev
output modalities, matching modality names case-insensitively. Explicit true
or false overrides remain authoritative, while a missing store or catalogue
record conservatively disables image output.

Wire the resolver through provider base configuration, update schema and model
documentation, and cover catalogue hits, misses, nil stores, and override
precedence.

Author: aheritier

agent-schema.json

@@ -1742,7 +1742,7 @@
         },
         "output_capabilities": {
           "$ref": "#/definitions/OutputCapabilitiesConfig",
-          "description": "Explicit, owner-declared generative output capabilities for this model. Never inferred from the model name or any catalogue — omit it, or leave a flag unset/false, to preserve existing behavior. Consumed by capability-gated request contracts (e.g. the Gemini gateway image-output route) that must not activate for a model unless its owner has affirmatively opted it in. Cannot be combined with first_available (set output_capabilities.image on the candidate models instead)."
+          "description": "Optional generative output capability override for this model. When omitted, Docker Agent detects output modalities from the models.dev catalogue. An explicit flag is authoritative, including false. Cannot be combined with first_available (set output_capabilities.image on the candidate models instead)."
         },
         "cost": {
           "$ref": "#/definitions/CostConfig",
@@ -1776,11 +1776,11 @@
     },
     "OutputCapabilitiesConfig": {
       "type": "object",
-      "description": "Explicit, owner-declared generative output capabilities for a model. There is no automatic detection — no catalogue of output-capable models exists, and matching on the model name string is deliberately avoided as unreliable — so this is nil/omitted by default. Omitted or false preserves existing behavior; only an explicit true opts a model into output-capability-gated behavior.",
+      "description": "Generative output capability overrides for a model. When omitted, Docker Agent detects output modalities from the models.dev catalogue. Explicit flags are authoritative, including false, and are useful for custom models or correcting catalogue metadata.",
       "properties": {
         "image": {
           "type": "boolean",
-          "description": "Whether the model is declared able to generate image output. Never inferred; must be set explicitly by the owner."
+          "description": "Whether the model can generate image output. When omitted, Docker Agent uses models.dev output modalities; an explicit value overrides the catalogue."
         }
       },
       "additionalProperties": false

pkg/model/provider/base/base.go

@@ -1,6 +1,8 @@
 package base
 
 import (
+	"context"
+
 	"github.com/docker/docker-agent/pkg/config/latest"
 	"github.com/docker/docker-agent/pkg/environment"
 	"github.com/docker/docker-agent/pkg/model/provider/options"
@@ -77,6 +79,16 @@ func (c *Config) CapsOverride() *modelinfo.CapsOverride {
 	return &modelinfo.CapsOverride{Image: caps.Image, PDF: caps.PDF, Audio: caps.Audio, Video: caps.Video}
 }
 
+// ImageOutputEnabled resolves the model's image-output capability from its
+// explicit tri-state configuration and, when unset, the models.dev catalogue.
+func (c *Config) ImageOutputEnabled(ctx context.Context) bool {
+	var override *bool
+	if caps := c.ModelConfig.OutputCapabilities; caps != nil {
+		override = caps.Image
+	}
+	return modelinfo.ResolveOutputImage(ctx, c.ModelOptions.ModelsDevStore(), c.ID(), override)
+}
+
 // EmbeddingResult contains the embedding and usage information
 type EmbeddingResult struct {
 	Embedding   []float64

pkg/modelinfo/modelinfo.go

@@ -639,7 +639,42 @@ func LoadCaps(ctx context.Context, store *modelsdev.Store, id modelsdev.ID) Mode
 	return capsFromModalities(model.Modalities.Input)
 }
 
-// capsFromModalities maps a models.dev input-modality list to the capability
+// ResolveOutputImage applies an explicit image-output override when present;
+// otherwise it derives support from the models.dev output modalities. Missing
+// catalogue data conservatively disables image output.
+func ResolveOutputImage(ctx context.Context, store *modelsdev.Store, id modelsdev.ID, override *bool) bool {
+	if override != nil {
+		return *override
+	}
+	if store == nil {
+		return false
+	}
+
+	ctx, cancel := context.WithTimeout(ctx, loadCapsTimeout)
+	defer cancel()
+
+	model, err := store.GetModel(ctx, id)
+	if err != nil {
+		if ctx.Err() != nil {
+			slog.WarnContext(ctx, "modelinfo: models.dev output lookup timed out, disabling image output",
+				"model", id.String(), "timeout", loadCapsTimeout)
+		} else {
+			warnCapsLookupMiss(ctx, id, err)
+		}
+		return false
+	}
+	return hasOutputModality(model.Modalities.Output, "image")
+}
+
+func hasOutputModality(modalities []string, expected string) bool {
+	for _, modality := range modalities {
+		if strings.EqualFold(modality, expected) {
+			return true
+		}
+	}
+	return false
+}
+
 // booleans it grants. Unknown modality names are ignored.
 func capsFromModalities(input []string) ModelCapabilities {
 	var mc ModelCapabilities