fix: Handling of site connection issues during outage

[rippyboii]

Summary

This PR improves bot startup reliability and moderator visibility when extensions/cogs fail to load.

• Aggregate extension + cog load failures during startup and report them as a single alert in #mod-log.
• Add retry + exponential backoff for cogs that depend on external sites/APIs (e.g., temporary 5xx/429/timeouts), with clear mod notifications on final failure.
• Add unit tests to validate retry behavior, error classification, and startup reporting.

During setup_hook, extensions are loaded concurrently. When an extension/cog fails due to a transient outage (rate limits, 5xx, timeouts), failures can either:

• stop startup unexpectedly, or
• fail noisily/fragmentedly, making it hard to see what broke and why.

This change standardizes both resilience (retry when appropriate) and visibility (one clean startup report + targeted alerts).

Changes

1) Startup failure aggregation (single #mod-log alert)

• Added utils/startup_reporting.py to format a standardized startup failure message.
• Updated bot.py to:
  • collect extension + cog load failures (import/setup/add_cog)
  • wait for all load tasks to complete
  • send one aggregated alert summarizing all failures
• Reporting is defensive: it does not crash if the log channel is unavailable.
• Startup continues for non-critical failures.

2) Retry + backoff for external/API-dependent cogs

Implemented retry logic with exponential backoff and explicit “retriable vs non-retriable” classification, plus moderator notifications when retries are exhausted.

Covered cogs include:

• Filtering: 3 attempts with backoff (1s, 2s, 4s); retries on HTTP 429, HTTP 5xx, TimeoutError, OSError; final failure logs + alerts #mod-alerts.
• Reminders: retry count is configured via URLs.connect_max_retries; warns are logged to Sentry; final failure posts to #mod-log.
• PythonNews: retries on 408, 429, 5xx, TimeoutError, OSError; on max retries logs + alerts mod_alerts and re-raises to stop startup.
• Superstarify cogs: added retry + notification and corresponding tests.

Tests / Verification

• Added unit tests covering:
  • retry-then-success
  • max-retries then alert + failure behavior
  • non-retriable errors
  • retry classification logic
  • aggregated startup failure reporting for faulty extensions/cogs

Suggested checks:

1. uv run task test
2. Run the bot and simulate a faulty extension/cog load to confirm a single aggregated #mod-log startup alert.

Moderator Alert in Discord:

Closes #2918

[rippyboii]

Hello @jb3,

I have opened another PR to keep the things clean. The earlier PR had referenced some commits including reports, which was the part of our assignment but not of this project. This new PR doesn't include any commits refrencing the reports or anything else that's unnecessary to this project.

All the suggestions from the previous PR are adapted in this PR too.

Apologies for delay update from our side.