test: add ha-ffmpeg for the tts platform's import chain

custom_components/openai_compatible/tts.py imports homeassistant.components.tts,
which imports homeassistant.components.ffmpeg, which does
`from haffmpeg.core import HAFFmpeg` at module level. Because
async_forward_entry_setups imports all of PLATFORMS (ai_task, conversation,
stt, tts) before setting any of them up, this one missing package failed
config entry setup entirely (SETUP_ERROR) and cascaded into the bulk of the
previous run's 104 failures: "Agent ... not found" in test_conversation.py,
KeyError('ai_task') in test_ai_task.py, and entry_not_loaded aborts /
KeyError('step_id') in test_config_flow.py's subentry flow tests.

Pinned to core tag 2026.9.2's homeassistant/components/ffmpeg/manifest.json.

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

requirements_test.txt

@@ -11,3 +11,4 @@ gazetteer-matcher==1.1.0
 home-assistant-intents==2026.8.28
 mutagen==1.48.1
 PyTurboJPEG==1.8.3
+ha-ffmpeg==3.2.2