[boly38]

Summary

• Add an optional, isolated Opper API reachability check to Telegram /health,
  enabled with TELEGRAM_HEALTH_OPPER_ENABLED=true and disabled by default.
• Use an unauthenticated GET with a five-second socket timeout, no redirects,
  retries, response-body processing, or model inference.
• Provide periodic, low-noise progress updates without resetting the existing
  300-second request deadline.
• Surface correlated assistant/provider errors promptly with sanitized categories,
  HTTP status in diagnostics, and request references.
• Restore the OpenCode request phase after progress edits so subsequent failures
  are delivered correctly.

Scope and limitations

The check measures bridge-to-Opper API reachability only. It does not validate
OpenCode's network path, credentials, credits, or model inference availability.
DNS resolution remains subject to the system resolver's timeout.

No automatic preflight is added before prompts, no provider key is passed to the
bridge, and no OpenCode server changes or unsafe debug mode are included.

Responses containing both partial text and an error are reported as failures,
rather than presenting the partial text as a successful response. Partial text
and the cost footer are not delivered for these failed exchanges.

Verification

• make test: 86 tests passed.
• python3 -m compileall -q bot/telegram/bloc_bridge: passed.
• git diff --check: passed.
• make config: passed in OpenCode-only mode.
• TELEGRAM_ENABLED=true TELEGRAM_HEALTH_OPPER_ENABLED=true make config: passed.
• No deployment, live Telegram exchange, or model inference was performed.

Closes #25