fix: restore message history loading

[Aryan-Verma-999]

fix: resolve 400 Bad Request and restore message loading for public channels

Acceptance Criteria fulfillment

• Room-scoped state flags (isChannelPrivate, isRoomTeam, isChannelReadOnly, isChannelArchived) correctly reset when switching between channels.
• API class dynamically resolves the correct REST endpoint (channels.messages vs groups.messages) based on actual room data.
• Message history successfully loads for public channels (e.g., #general) even after opening a private room.

Fixes #1232

Description

Root Cause:
Previously, room state flags like isChannelPrivate in the Zustand store were only conditionally updated to true when encountering a private room (t === 'p'). Because they were never explicitly reset to false, switching to a public room later would falsely preserve the private state. This caused the EmbeddedChatApi to call the groups.* REST endpoints instead of channels.* for public rooms, resulting in a 400 Bad Request and failing to load message history.

Solution:

1. Frontend State Fix: We now derive and explicitly set isChannelPrivate and other boolean flags dynamically (setIsChannelPrivate(res.room.t === 'p')) upon every room switch in ChatHeader.js

2. API Layer Hardening: added a resolveRoomRoute() mechanism to EmbeddedChatApi.ts, which enforces resolution of the correct endpoint ("groups" vs "channels") based on API-level roomInfo caching rather than blindly trusting frontend booleans.

PR Test Details

Note: The PR will be ready for live testing at https://rocketchat.github.io/EmbeddedChat/pulls/pr-1234 after approval. Contributors are requested to replace <pr_number> with the actual PR number.