fix(line): map LINE mentions to At elements so the at-bot rule works

[leonoxo]

Summary

The LINE adapter never parsed message.mentions: the full text (including
the @BotName chip text) went into a single Plain component. As a result,
the at: true group respond rule could never match on LINE — atbot.py
looks for an At element whose target equals bot_account_id, but the
adapter produced no At elements at all.

Users whose bot name is not a configured prefix therefore had to rely on
prefix/regexp rules to get the bot to respond to a LINE group @-mention, and
there was no log of the drop (the message is silently interrupted).

This PR maps LINE mentions to At elements:

• a mention of the bot itself (mentionee.is_self) → At(target=bot_account_id)
  — matches the existing at-bot rule directly;
• a mention of anyone else → At(target=user_id, display=name) — the
  @name text is preserved for prefix/regexp rules (At.__str__() renders
  @{display}, and display is already @-stripped, so no double @).

The text around mentions is split by mentionee.index/length (LINE
guarantees text[index:index+length] == '@name'); malformed or out-of-range
mention positions are skipped and the remainder falls back to plain text.
No-mention messages, images, and other message types are unchanged.

Related

Same area as #2398 (LINE group session identity); complements it — that one
fixed which session a group message goes to, this one fixes whether the
group message triggers a response at all.

Test Plan

• New unit tests in tests/unit_tests/platform/test_line_session_identity.py:
  • bot mention → At(target=bot_account_id) (the at-bot rule case)
  • mention of another user → At(target=user_id, display=name)
  • @ALL → plain text (unchanged behaviour)
  • multiple mentions → correct split with surrounding text
  • out-of-range mention indices → graceful fallback to plain text
  • no mention → single plain component (regression guard)
• Existing session-identity tests updated to the instance-method converter.
• uv run pytest tests/unit_tests/ → 2803 passed
• uv run ruff check / ruff format --check clean

[codecov]

Codecov Report

❌ Patch coverage is 91.42857% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/langbot/pkg/platform/sources/line.py	91.42%	3 Missing ⚠️

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