chore(api): clarify Realtime modality array definitions (openai#3954)

## Summary

Clarifies retained Realtime modality arrays in the API reference without
changing SDK types or method signatures.

## Changes

- Explicitly describe retained Realtime modality fields as arrays of
text or audio values.
- Preserve existing optionality and SDK behavior.

Co-authored-by: markstuart-oai <323302876+markstuart-oai@users.noreply.github.com>

Author: markstuart-oai

.castiron.stats.yml

@@ -1,8 +1,8 @@
 schema_version: 1
-generation_id: 7e63eb4a-5670-4cc0-b884-4076bfc986bf
-openapi_spec_hash: 4bba843ccb59994fa67b0d84129af8ae
-openapi_transformed_spec_hash: 52080a33f224bf8e7670301604d34c7e
+generation_id: 4bda3ef4-13a5-473f-b0cd-6fa6b69a0db8
+openapi_spec_hash: 6fbcb590bdf88b5d464546b290ab35e7
+openapi_transformed_spec_hash: 4e4a94f25153ab253178d6c098cb4f0e
 config_hash: 84cdcc959ee7140cc9a65e9b642c097b
-codegen_sha: 51f0864c8108de1eaf5a823e8e152495885e8191
+codegen_sha: c11ad8ad6a402c5cabfff79093c308035fe39644
 codegen_hash: dc4765fced94aa65178e5fbda91d2844a712ac8d8e8b7fd8993fde98dbb1cb04
-public_codegen_sha: 65730dc4902028c25ee17020a1ea67f82e7e9658
+public_codegen_sha: d54ac7ae41705340870935d4d6484843cb5d9d29

api_reference/openapi.transformed.yml

@@ -54156,6 +54156,7 @@ components:
           - realtime.session
           description: The object type. Always `realtime.session`.
         modalities:
+          type: array
           description: |
             The set of modalities the model can respond with. To disable audio,
             set this to ["text"].
@@ -54369,6 +54370,7 @@ components:
           - value
           - expires_at
         modalities:
+          type: array
           description: |
             The set of modalities the model can respond with. To disable audio,
             set this to ["text"].