feat(telegram): surface file metadata and support document downloads

[dorey-agent]

Motivation

When a user sends a document (PDF, audio, video, etc.) via Telegram, the agent only receives a generic label like [📎 Document]. No file name, size, or mime type is passed through. The agent cannot acknowledge what was sent, let alone read it.

Photos are already downloaded and forwarded — other attachment types either fail silently or produce no useful metadata.

Closes #725

Solution

Always surface file metadata regardless of whether the file is downloaded:

• FileMetadata struct carries filename, mime_type, file_size extracted from Telegram message objects
• When download is skipped (file too large) or fails, the agent receives a rich label:
  [📎 Document (name=report.pdf, type=application/pdf, size=25.3MB)]
• When download succeeds, the agent receives the full ContentPart::File/Audio/Image as before

Configurable size threshold via media_max_mb option (default: 20MB, Telegram Bot API hard limit):

• Files exceeding the limit are not downloaded but metadata is still surfaced
• Configurable per-channel in anyclaw.yaml options

Extended media type support:

• Video and animation (GIF) attachments are now downloaded and forwarded (previously fell through to unresolved text label)
• All unsupported types (sticker, location, contact, video_note) now surface available metadata via extract_generic_metadata()

Design decisions:

• media_max_bytes uses AtomicU64 since SharedState is behind Arc (shared across dispatcher + adapter)
• Size check happens before bot.get_file() call to avoid unnecessary API calls
• Follows OpenClaw pattern: always metadata, conditionally download

Testing

cargo test --manifest-path ext/Cargo.toml -p channel-telegram   # 257 unit + 6 integration tests pass
cargo clippy --manifest-path ext/Cargo.toml -p channel-telegram  # zero warnings
cargo +1.95.0 fmt --check --all --manifest-path ext/Cargo.toml  # clean

Key tests:

• when_process_media_video_then_sends_video_label — video routing
• when_document_message_with_caption_then_sends_label_and_caption — metadata label formatting
• when_process_media_without_caption_then_sends_placeholder — unresolved metadata stub

Checklist

• Tests pass (cargo test)
• No clippy warnings (cargo clippy --workspace -- -D warnings)
• Formatted (cargo fmt --all -- --check)
• PR title follows Conventional Commits format
• Updated CHANGELOG.md if this is a user-facing binary change (version bump 0.12.2 → 0.13.0)

[ai-assisted]

[dorey-agent]

Automated Review Summary

Agents: correctness, edge_cases
Findings: 🔴 0 critical | 🟡 0 warning | 💬 3 nit

---

ext/channels/telegram/src/dispatcher.rs:854
💬 [correctness] VideoNote metadata was missing — fixed in follow-up commit d2b25a4.

ext/channels/telegram/src/adapter.rs:733
💬 [edge_cases] Setting media_max_mb=0 silently blocks all downloads. Acceptable behavior (metadata-only mode) but could document this in defaults.yaml.

ext/channels/telegram/src/dispatcher.rs:609
💬 [edge_cases] When Telegram reports file.size=0 (unknown size), the download proceeds without a streaming cap. Acceptable for now since Telegram Bot API hard-caps at 20MB anyway.