feat: vendor openai_conversation from core 2026.9.2, rename domain

Verbatim copy from home-assistant/core at tag 2026.9.2, with only the
domain constant and manifest changed. Apache-2.0, attribution in NOTICE.

Also fixed self-references the domain rename left dangling: the
config_entry selector's `integration:` field in services.yaml, the
`[%key:component::openai_conversation::...%]` translation
cross-references in strings.json, the deprecated-action descriptions
naming the old domain, and config_flow.py's instructions_url (now
pointing at this repo instead of the core docs page).

Local pytest run skipped per owner decision (see prior commit); files
validated with py_compile / json.load / yaml.safe_load instead. CI
(added in the prior commit) is the real gate.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01739Seac5eMAfNwFeiwV5r4

custom_components/openai_compatible/conversation.py

@@ -0,0 +1,90 @@
+"""Conversation support for OpenAI."""
+
+from typing import Literal, override
+
+from homeassistant.components import conversation
+from homeassistant.config_entries import ConfigSubentry
+from homeassistant.const import CONF_LLM_HASS_API, CONF_PROMPT, MATCH_ALL
+from homeassistant.core import HomeAssistant
+from homeassistant.helpers.entity_platform import AddConfigEntryEntitiesCallback
+
+from . import OpenAIConfigEntry
+from .const import DOMAIN
+from .entity import OpenAIBaseLLMEntity
+
+PARALLEL_UPDATES = 0
+
+
+async def async_setup_entry(
+    hass: HomeAssistant,
+    config_entry: OpenAIConfigEntry,
+    async_add_entities: AddConfigEntryEntitiesCallback,
+) -> None:
+    """Set up conversation entities."""
+    for subentry in config_entry.subentries.values():
+        if subentry.subentry_type != "conversation":
+            continue
+
+        async_add_entities(
+            [OpenAIConversationEntity(config_entry, subentry)],
+            config_subentry_id=subentry.subentry_id,
+        )
+
+
+class OpenAIConversationEntity(
+    conversation.ConversationEntity,
+    conversation.AbstractConversationAgent,
+    OpenAIBaseLLMEntity,
+):
+    """OpenAI conversation agent."""
+
+    _attr_supports_streaming = True
+
+    def __init__(self, entry: OpenAIConfigEntry, subentry: ConfigSubentry) -> None:
+        """Initialize the agent."""
+        super().__init__(entry, subentry)
+        if self.subentry.data.get(CONF_LLM_HASS_API):
+            self._attr_supported_features = (
+                conversation.ConversationEntityFeature.CONTROL
+            )
+
+    @property
+    @override
+    def supported_languages(self) -> list[str] | Literal["*"]:
+        """Return a list of supported languages."""
+        return MATCH_ALL
+
+    @override
+    async def async_added_to_hass(self) -> None:
+        """When entity is added to Home Assistant."""
+        await super().async_added_to_hass()
+        conversation.async_set_agent(self.hass, self.entry, self)
+
+    @override
+    async def async_will_remove_from_hass(self) -> None:
+        """When entity will be removed from Home Assistant."""
+        conversation.async_unset_agent(self.hass, self.entry)
+        await super().async_will_remove_from_hass()
+
+    @override
+    async def _async_handle_message(
+        self,
+        user_input: conversation.ConversationInput,
+        chat_log: conversation.ChatLog,
+    ) -> conversation.ConversationResult:
+        """Process the user input and call the API."""
+        options = self.subentry.data
+
+        try:
+            await chat_log.async_provide_llm_data(
+                user_input.as_llm_context(DOMAIN),
+                options.get(CONF_LLM_HASS_API),
+                options.get(CONF_PROMPT),
+                user_input.extra_system_prompt,
+            )
+        except conversation.ConverseError as err:
+            return err.as_conversation_result()
+
+        await self._async_handle_chat_log(chat_log)
+
+        return conversation.async_get_result_from_chat_log(user_input, chat_log)