feat: add thread_id and local file support to proactive channel_send

[houko]

Summary

• add thread_id and file_path support to proactive channel_send
• route proactive channel sends through send_in_thread(...) when thread_id is present
• let Telegram sendDocument accept an existing local file path and upload it via multipart while preserving message_thread_id
• preserve file captions end-to-end for proactive file sends
• make file_path explicitly telegram-only for now, with a clear runtime error on unsupported channels
• add tests for schema exposure, kernel handoff, local path detection, caption propagation, and multipart request behavior

Why

Closes #509. OpenFang already supports threaded Telegram replies at the adapter level, but the proactive outbound path still could not target a specific thread/topic or send a generated local file without an external wrapper.

Validation

• cargo test -p openfang-runtime test_channel_send
• cargo test -p openfang-channels telegram::tests::test_api_send_document_multipart_for_local_file_preserves_thread_id
• cargo test -p openfang-channels telegram::tests::test_parse_telegram_document_fallback
• cargo test -p openfang-channels telegram::tests::test_local_document_path_accepts_existing_file_and_file_scheme
• cargo check -p openfang-kernel
• live-fire verification on a real Telegram topic: sent text and a local PDF with caption to topic 18 using the PR branch code

[houko]

Follow-up pushed in e084f8c.

This tightens the initial implementation in two ways:

• preserves file captions end-to-end for channel_send -> kernel -> channel adapters
• makes file_path explicitly telegram-only for now, with a clear runtime error for other channels

Validation rerun on the PR branch:

• cargo test -p openfang-runtime test_channel_send
• cargo test -p openfang-channels telegram::tests::test_api_send_document_multipart_for_local_file_preserves_thread_id
• cargo test -p openfang-channels telegram::tests::test_parse_telegram_document_fallback
• cargo check -p openfang-kernel

Also re-verified live against a real Telegram topic on the PR branch code:

• sent text to topic 18
• sent a local PDF to topic 18 with caption PR #510 caption pipeline check

[houko]

Follow-up pushed in 8cf0a24.

This also fixes Telegram rendering for proactive outbound sends:

• channel_send text now uses the same channel formatter defaults as bridge replies
• Telegram proactive messages render Markdown-style **bold**, *italic*, `code`, and links as Telegram HTML instead of showing raw markup
• image/file captions in the proactive path now go through the same formatter

Validation:

• cargo test -p openfang-kernel test_format_proactive_channel_text

[houko]

Added two follow-up commits on top of the existing thread/topic work:

• 796b51a feat(telegram): support local image attachments in proactive sends
  • adds image_path to channel_send
  • routes local Telegram images through multipart sendPhoto
  • preserves message_thread_id for proactive topic delivery
• ce0e969 feat(formatter): render task lists for telegram
  • renders Markdown task lists like - [x] / - [ ] as ☑ / ☐ in Telegram formatting
  • keeps nested list bullets from being misparsed as italics

Validation rerun after these commits:

• cargo test -p openfang-runtime test_channel_send
• cargo test -p openfang-channels formatter::tests::
• cargo test -p openfang-channels test_api_send_

The sendPhoto coverage now includes a multipart local-file test that checks message_thread_id is preserved.

[jaberjaber23]

Has merge conflicts with current main. Please rebase and resubmit.