fix(line): implement create_source for dispatch

[KevinJ1729]

Summary

• implement LineAdapter.create_source() directly in the native LINE platform plugin
• pass the inbound LINE message_id into the generated SessionSource
• add regression coverage for LINE SessionSource creation and message dispatch

Context

Hermes Agent v0.13.0 includes the native LINE platform plugin, but LINE webhook dispatch can fail when the gateway calls the adapter source-construction contract and the LINE adapter does not provide an explicit create_source() implementation.

In production this showed up after signed LINE webhook events reached the adapter, then failed before dispatch with:

AttributeError: 'LineAdapter' object has no attribute 'create_source'

This patch makes the LINE adapter's dispatch contract explicit instead of relying on surrounding gateway/base behavior, so LINE webhook events consistently produce a SessionSource with platform=line, chat/user metadata, and the triggering message id.

Testing

• python -m py_compile plugins/platforms/line/adapter.py tests/gateway/test_line_plugin.py
• Could not run python -m pytest tests/gateway/test_line_plugin.py -q locally because pytest is not installed in this Windows Python environment.
• Could not run uv run pytest tests/gateway/test_line_plugin.py -q locally because uv is not installed in this Windows shell.

[alt-glitch]

Duplicate of #23305 (and #23730, #23782, #23867, #24091). This is at least the 6th PR addressing the same LINE create_source → build_source issue. The approach here (compatibility shim) differs slightly but addresses the same root cause.

[KevinJ1729]

Thanks for the clarification. I understand this PR overlaps with the existing fixes for the LINE create_source/build_source issue.

To avoid adding noise for maintainers, I’ll close this PR. If the message_id handling or regression test from this PR is still useful after the main fix lands, I’d be happy to open a smaller follow-up PR focused only on that part.