Limit slack block text length when rendering alert group timeline (#5246)

# What this PR does
Limit length of text in block being posted to slack when showing alert
group timeline.

## Which issue(s) this PR closes

Related to [issue link here]

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## Checklist

- [x] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] Added the relevant release notes label (see labels prefixed w/
`release:`). These labels dictate how your PR will
    show up in the autogenerated release notes.

engine/apps/slack/scenarios/alertgroup_timeline.py

@@ -2,6 +2,7 @@ import typing
 
 from apps.api.permissions import RBACPermission
 from apps.slack.chatops_proxy_routing import make_private_metadata
+from apps.slack.constants import BLOCK_SECTION_TEXT_MAX_SIZE
 from apps.slack.scenarios import scenario_step
 from apps.slack.scenarios.slack_renderer import AlertGroupLogSlackRenderer
 from apps.slack.types import (
@@ -47,9 +48,13 @@ class OpenAlertGroupTimelineDialogStep(AlertGroupActionsMixin, scenario_step.Sce
         future_log_report = AlertGroupLogSlackRenderer.render_alert_group_future_log_report_text(alert_group)
         blocks: typing.List[Block.Section] = []
         if past_log_report:
-            blocks.append({"type": "section", "text": {"type": "mrkdwn", "text": past_log_report}})
+            blocks.append(
+                {"type": "section", "text": {"type": "mrkdwn", "text": past_log_report[:BLOCK_SECTION_TEXT_MAX_SIZE]}}
+            )
         if future_log_report:
-            blocks.append({"type": "section", "text": {"type": "mrkdwn", "text": future_log_report}})
+            blocks.append(
+                {"type": "section", "text": {"type": "mrkdwn", "text": future_log_report[:BLOCK_SECTION_TEXT_MAX_SIZE]}}
+            )
 
         view: ModalView = {
             "blocks": blocks,