Stream OpenClaw responses through native Assist

[techartdev]

Reimplements #30 on the current integration and Home Assistant conversation API instead of merging stale #21.

• Registers the integration as a ConversationEntity and advertises native streaming.
• Streams OpenClaw SSE deltas into Home Assistant's chat log on HA 2025.8+.
• Retains the prior non-streaming behavior on HA 2025.1–2025.7 for compatibility.
• Falls back to a regular chat-completions response when the SSE stream is empty or fails.
• Adds dependency-free coverage for streaming deltas, fallback, and legacy mode.

Validation:

• python3 -m unittest discover -s tests -v (12 tests)
• Python syntax check
• Chat-card JS syntax check

Closes #30.

[techartdev]

Live validation found two blockers before merge: (1) OpenClaw Gateway buffers generation and emits all SSE chunks only at the end, so this does not currently reduce TTS latency; (2) the class inherits conversation.ConversationEntity unconditionally, so the claimed HA 2025.1-2025.7 fallback cannot run if that symbol is absent during module import. Keeping the PR open as the active experiment, but it needs a Gateway-side streaming fix plus a real import-compatible legacy strategy (or a raised minimum HA version) before release.