fix(line): use stable source id for session identity

[leonoxo]

Summary

• LINEEventConverter.target2yiri() built Friend.id/Group.id (and the GroupMember sender id) from event.message.id, which is unique per message — every incoming message therefore mapped to a different session key, so LINE users and groups lost conversation context on every single turn.
• Switched to event.source.user_id/.group_id/.room_id, the stable identifiers LINE actually provides for this purpose, matching the pattern already used by other adapters (e.g. telegram.py uses event.effective_chat.id).
• Falls back to the group/room id when user_id is absent, per LINE's documented behavior ("Only included in message events. Only users of LINE for iOS and LINE for Android are included in userId") for some group/room members.

Test plan

• Added tests/unit_tests/platform/test_line_session_identity.py covering: stable sender id across multiple messages from the same user, stable group id across multiple messages, and room-type fallback when user_id is missing.
• uv run pytest tests/unit_tests/platform/test_line_session_identity.py tests/unit_tests/platform/test_line_limits.py -q — 6 passed
• uv run ruff check / ruff format --check on changed files

[github-actions]

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[leonoxo]

I have read the CLA Document and I hereby sign the CLA

[RockChinQ]

Hi @leonoxo , would you provide some screenshots to prove the changes work?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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