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/services.yaml

@@ -0,0 +1,60 @@
+generate_image:
+  fields:
+    config_entry:
+      required: true
+      selector:
+        config_entry:
+          integration: openai_compatible
+    prompt:
+      required: true
+      selector:
+        text:
+          multiline: true
+    size:
+      required: false
+      example: "1024x1024"
+      default: "1024x1024"
+      selector:
+        select:
+          options:
+            - "1024x1024"
+            - "1024x1792"
+            - "1792x1024"
+    quality:
+      required: false
+      example: "standard"
+      default: "standard"
+      selector:
+        select:
+          options:
+            - "standard"
+            - "hd"
+    style:
+      required: false
+      example: "vivid"
+      default: "vivid"
+      selector:
+        select:
+          options:
+            - "vivid"
+            - "natural"
+generate_content:
+  fields:
+    config_entry:
+      required: true
+      selector:
+        config_entry:
+          integration: openai_compatible
+    prompt:
+      required: true
+      selector:
+        text:
+          multiline: true
+      example: "Hello, how can I help you?"
+    filenames:
+      selector:
+        text:
+          multiline: true
+      example: |
+        - /path/to/file1.txt
+        - /path/to/file2.txt