fix(telegram): harden photo download against crash scenarios

[snemesh]

Problem

Sending a photo to the Telegram bot can crash the channel process, making it completely unresponsive until restarted (#944). The photo download callback in message:photo handler has three gaps:

1. No empty array guard — if ctx.message.photo is empty, photos[photos.length - 1] is undefined and .file_id throws a TypeError that propagates as an unhandled error
2. No HTTP status check — if the Telegram file API returns 4xx/5xx, res.arrayBuffer() reads an error body that gets written to disk as a corrupt JPEG
3. No size limit — without a cap, a large file response could cause memory pressure

Fix

All three cases now return undefined (graceful degradation) instead of crashing. The message is still delivered to Claude without the image attachment, and errors are logged to stderr.

Changes

• external_plugins/telegram/server.ts — 10 insertions, 1 deletion

Test plan

• bun build passes
• 12/12 resilience tests pass — covers all crash scenarios

Test output (12/12 passed)

── Normal flow ──
  ✅ Normal photo download → /tmp/inbox/...-u1.jpg

── Crash scenario: empty photo array ──
  ✅ Empty array → no crash → undefined (graceful)
  ✅ null photos → no crash → undefined (graceful)
  ✅ undefined photos → no crash → undefined (graceful)

── Crash scenario: Telegram API edge cases ──
  ✅ No file_path → graceful
  ✅ getFile throws → graceful

── Crash scenario: HTTP errors ──
  ✅ HTTP 404 → graceful
  ✅ HTTP 500 → graceful
  ✅ fetch failed → graceful

── Crash scenario: resource limits ──
  ✅ 60MB photo → rejected gracefully
  ✅ arrayBuffer() throws → graceful

── Normal flow: multiple sizes ──
  ✅ Picks largest (last) from array

═══ Result: 12/12 passed ✅ ═══

Fixes #944