feat(hooks): include text and message in AGENT_COMPLETE event

[Copilot]

AGENT_COMPLETE events carried usage metrics but not the assistant's final output, forcing consumers to separately reconstruct it. text and message are now always present in the payload — uniform contract, no optional-key guards needed.

Changes

src/strands_compose/hooks/event_publisher.py

• AGENT_COMPLETE data now always includes:
  • text — str(result) (flat string: handles text, citations, structured output) or "" when no result
  • message — result.message (full Message TypedDict: role, content blocks, metadata) or {} when no result

# AGENT_COMPLETE event data shape
{
    "type": "agent",
    "usage": {"input_tokens": ..., "output_tokens": ..., "total_tokens": ...},
    "text": "Final assistant response as plain string",
    "message": {
        "role": "assistant",
        "content": [...],   # text, toolUse, reasoningContent, citationsContent
        "metadata": {...},  # per-turn usage + metrics
    },
}

tests/unit/hooks/test_event_publisher.py

• Updated existing usage test to explicitly set result=None
• Added test: text/message populated correctly when result is present
• Added test: text=""/message={} defaults when result is None

[galuszkm]

lgtm