[Codex][CLI] Gate image inputs by model modalities (#10271)

###### Summary

- Add input_modalities to model metadata so clients can determine
supported input types.
- Gate image paste/attach in TUI when the selected model does not
support images.
- Block submits that include images for unsupported models and show a
clear warning.
- Propagate modality metadata through app-server protocol/model-list
responses.
  - Update related tests/fixtures.

  ###### Rationale

  - Models support different input modalities.
- Clients need an explicit capability signal to prevent unsupported
requests.
- Backward-compatible defaults preserve existing behavior when modality
metadata is absent.

  ###### Scope

  - codex-rs/protocol, codex-rs/core, codex-rs/tui
  - codex-rs/app-server-protocol, codex-rs/app-server
  - Generated app-server types / schema fixtures

  ###### Trade-offs

- Default behavior assumes text + image when field is absent for
compatibility.
  - Server-side validation remains the source of truth.

  ###### Follow-up

- Non-TUI clients should consume input_modalities to disable unsupported
attachments.
- Model catalogs should explicitly set input_modalities for text-only
models.

  ###### Testing

  - cargo fmt --all
  - cargo test -p codex-tui
  - env -u GITHUB_APP_KEY cargo test -p codex-core --lib
  - just write-app-server-schema
- cargo run -p codex-cli --bin codex -- app-server generate-ts --out
app-server-types
  - test against local backend
  
<img width="695" height="199" alt="image"
src="https://github.com/user-attachments/assets/d22dd04f-5eba-4db9-a7c5-a2506f60ec44"
/>

---------

Co-authored-by: Josh McKinney <joshka@openai.com>

codex-rs/app-server-protocol/schema/json/v2/ModelListResponse.json

@@ -1,6 +1,25 @@
 {
   "$schema": "http://json-schema.org/draft-07/schema#",
   "definitions": {
+    "InputModality": {
+      "description": "Canonical user-input modality tags advertised by a model.",
+      "oneOf": [
+        {
+          "description": "Plain text turns and tool payloads.",
+          "enum": [
+            "text"
+          ],
+          "type": "string"
+        },
+        {
+          "description": "Image attachments included in user turns.",
+          "enum": [
+            "image"
+          ],
+          "type": "string"
+        }
+      ]
+    },
     "Model": {
       "properties": {
         "defaultReasoningEffort": {
@@ -15,6 +34,16 @@
         "id": {
           "type": "string"
         },
+        "inputModalities": {
+          "default": [
+            "text",
+            "image"
+          ],
+          "items": {
+            "$ref": "#/definitions/InputModality"
+          },
+          "type": "array"
+        },
         "isDefault": {
           "type": "boolean"
         },