Forward app conversation observability fields

[juanmichelini]

Summary

• Add observability metadata, tags, and span name fields to AppConversationStartRequest
• Forward those API-provided observability fields into regular OpenHands conversation starts
• Forward the same fields through ACP conversation starts and deduplicate tags while preserving server metadata precedence

Port of OpenHands/sandbox-server#3 for enterprise.

Test plan

• uv run --frozen ruff check openhands/app_server/app_conversation/app_conversation_models.py openhands/app_server/app_conversation/live_status_app_conversation_service.py tests/unit/app_server/test_live_status_app_conversation_service.py
• PYTHONPATH=".:$PYTHONPATH" uv run --frozen pytest -q tests/unit/app_server/test_live_status_app_conversation_service.py -k "observability or app_conversation_start_request_accepts_observability_fields"

Note: poetry-based checks failed locally before project code loaded due to a Python 3.14/pyexpat dynamic-library issue (_XML_SetAllocTrackerActivationThreshold missing), so validation was run via uv with Python 3.13.

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Enterprise server image for this PR:

ghcr.io/openhands/enterprise-server:sha-88bd916

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
openhands/app_server/app_conversation
app_conversation_models.py
live_status_app_conversation_service.py					1661-1662, 1666-1671, 2016-2017, 2226-2247, 2381, 2527-2538, 2547-2552
Project Total

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