Fixes #8421

Problem

Slack sometimes sends events where the channel_type field contradicts the channel ID prefix. For example, a DM channel with ID D0ABC123 may arrive with channel_type: "channel" instead of "im". This causes the bot to misclassify DMs as public channels, leading to:

• Wrong session key: messages route to agent:main:slack:channel:D0ABC123 instead of agent:main:main (when dmScope is "main")
• Wrong ChatType: set to "channel" instead of "direct"
• Policy bypass: DM policy checks are skipped because the message is treated as a channel message
• Broken context: conversation history is isolated per-channel instead of unified in the DM session

Root Cause

normalizeSlackChannelType() in context.ts trusts the event's channel_type field when it's a recognized value ("im", "mpim", "channel", "group"), even when the channel ID prefix contradicts it. Since Slack channel IDs with D prefix are always DMs by Slack's own convention, the channel_type field should be overridden when it disagrees.

Fix

Added a cross-validation check in normalizeSlackChannelType(): when the channel ID starts with D (inferred as "im"), override any contradicting channel_type value. This is the canonical normalization function used across the entire Slack integration — all downstream callers (prepareSlackMessage, isChannelAllowed, resolveSlackSystemEventSessionKey, member events) are automatically protected.

Only D-prefix is overridden because it unambiguously identifies DMs. G-prefix is intentionally left alone since it's ambiguous (can be either a private channel or a group DM — Slack's API provides the correct channel_type in that case).

Test Plan

TDD approach — all 8 new tests were written first and confirmed failing before the fix:

• normalizeSlackChannelType unit tests (3 cases):
  • Overrides wrong channel_type ("channel", "group", "mpim") for D-prefix channels → returns "im"
  • Preserves correct channel_type ("im") for D-prefix channels
  • Does not override G-prefix channels (ambiguous prefix)
• prepareSlackMessage integration tests (2 cases):
  • D-prefix + wrong channel_type: "channel" → isDirectMessage: true, session routes to agent:main:main, ChatType: "direct"
  • D-prefix + missing channel_type → correctly inferred as DM
• All 5337 existing tests pass (no regressions)
• Full CI gate: pnpm build && pnpm check && pnpm test

Greptile Overview

Greptile Summary

• Updates normalizeSlackChannelType() to cross-check the channel ID prefix and override contradictory channel_type values for D* channels (treat as im).
• Adds unit tests covering D-prefix overrides, preserving correct values, and leaving ambiguous G* channels unchanged.
• Adds inbound prepareSlackMessage contract tests to ensure D-prefix DMs route to the main DM session and produce ChatType: direct even when Slack mislabels channel_type.

Confidence Score: 5/5

• This PR is safe to merge with minimal risk.
• Change is narrowly scoped to normalization logic, has clear test coverage for the regression scenario (D-prefix DM misclassified), and doesn’t affect ambiguous prefixes; no additional behavioral regressions were found in call sites using this normalization.
• No files require special attention