fix(drift): fix SplitCodeBlocks line duplication and handle plain text messages

[raman1236]

Summary

Fixes #745 - Drift alerts in Slack > 4000 characters break formatting

Bugs Fixed

1. Line duplication in SplitCodeBlocks

When a line caused the accumulated message to exceed Slack's 4000-character limit, the splitting logic appended the triggering line to both the current chunk and the start of the next chunk:

// Before (buggy):
res = append(res, part+"\n"+line+"\n```")  // line added to OLD part
part = "```\n" + line                       // line ALSO added to NEW part

This caused every line at a split boundary to appear twice in the Slack notification.

Fix: Close the current chunk cleanly without the triggering line, then start the new chunk with it:

// After:
res = append(res, part+"\n```")
part = "```\n" + line

2. Messages without code blocks skipped splitting

The function returned messages with fewer than 2 ``` markers unsplit, even when they exceeded 4000 characters. This caused the Slack API to reject them.

Fix: Remove the early return and add a plain-text splitting path.

3. SendErrorNotificationForProject didn't split messages

Unlike SendNotificationForProject, the error notification method sent messages directly via SendSlackMessage without going through SplitCodeBlocks. Long error stack traces would be rejected by Slack.

Fix: Use the same split-and-loop pattern.

Testing

Added 2 new unit tests:

• TestSplitCodeBlocksNoLineDuplication — verifies no line appears in multiple parts after splitting
• TestSplitCodeBlocksPlainTextOver4000 — verifies plain text messages > 4000 chars are split

All existing tests continue to pass:

=== RUN   TestSlackSplitLargerMessage      --- PASS
=== RUN   TestSlackSmallerMessageNotSplit   --- PASS
=== RUN   TestSplitCodeBlocksNoLineDuplication --- PASS
=== RUN   TestSplitCodeBlocksPlainTextOver4000 --- PASS