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/UPSTREAM_COMMIT

@@ -0,0 +1 @@
+33c3e0cca60e73a8c4970ee677d75b8bc6464cdf