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/manifest.json

@@ -0,0 +1,14 @@
+{
+  "domain": "openai_compatible",
+  "name": "OpenAI Compatible",
+  "after_dependencies": ["assist_pipeline", "intent"],
+  "codeowners": ["@adamjs83"],
+  "config_flow": true,
+  "dependencies": ["conversation"],
+  "documentation": "https://github.com/adamjs83/ha-openai-compatible",
+  "integration_type": "service",
+  "iot_class": "cloud_polling",
+  "issue_tracker": "https://github.com/adamjs83/ha-openai-compatible/issues",
+  "requirements": ["openai==2.45.0"],
+  "version": "1.0.0"
+}