feat(api): provide self-hosted guidance message for WorkspaceMemberLimitExceeded error

[tonicofonico]

📝 Description of Changes

Fixes #1641.

This PR updates the error message string for AppError::WorkspaceMemberLimitExceeded (ErrorCode::WorkspaceMemberLimitExceeded / 1027) to simplify backend error reporting.

🔗 Related Pull Requests & Issues

• Issue feat(console): improve Community edition UX guidance when workspace seat limit is reached #1641: UX improvement tracking workspace member limit guidance on self-hosted instances.
• PR docs: add self-hosted whitelist onboarding and reverse proxy authentication guide #1643: Comprehensive self-hosted documentation and deployment guide.
• PR fix(deploy): add runtime admin authentication fallback #1637: Added NEXT_PUBLIC_DISABLE_SERVER_ACTIONS option for reverse proxy deployments.
• PR feat(auth): auto-assign super/system admin metadata for GoTrue users via database migration #1639 & Issue feat(auth): auto-assign super/system admin metadata for GOTRUE_ADMIN_EMAIL on initial setup #1638: PostgreSQL migration for automatic super admin role synchronization.
• PR fix(deploy): add healthcheck to appflowy_search and set service_healthy dependency for appflowy_cloud #1640: Docker Compose startup health check ordering for appflowy_search.

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🔍 Problem & Motivation

On Community Self-Hosted deployments, member seats per workspace are limited to 1, but user onboarding is managed via Signup Settings / Whitelist (/console/users-management?tab=settings).

Previously, when member limits were hit during workspace invitation, users received a generic billing error without actionable guidance on how to enable self-hosted team access.

🛠️ Changes Included:

• libs/app-error/src/lib.rs: Added explicit AppError::WorkspaceMemberLimitExceeded variant mapping cleanly to ErrorCode::WorkspaceMemberLimitExceeded (1027).

🧪 Verification:

• Verified AppError::WorkspaceMemberLimitExceeded maps cleanly to ErrorCode::WorkspaceMemberLimitExceeded (1027).
• Verified JSON response formatting.

Summary by Sourcery

Bug Fixes:

• Provide a clearer, workspace-specific member limit error message to improve guidance for self-hosted deployments.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Adds a dedicated WorkspaceMemberLimitExceeded app error variant and ensures it maps to the corresponding error code to support clearer self-hosted guidance messaging when workspace member limits are reached.

File-Level Changes

Change	Details	Files
Introduce a WorkspaceMemberLimitExceeded app error and wire it into the central error-code mapping.	Add WorkspaceMemberLimitExceeded variant to the AppError enum with a specific user-facing error message. Update AppError::to_error_code (or equivalent mapping method) to return ErrorCode::WorkspaceMemberLimitExceeded for the new variant.	libs/app-error/src/lib.rs

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#1641	Detect when the server is operating under a Community Self-Hosted license so that specific guidance can be shown when the workspace member seat limit is reached.	❌	The PR only adds a new AppError variant (WorkspaceMemberLimitExceeded) and maps it to an existing error code. There is no logic for detecting Community Self-Hosted licenses or any conditional behavior based on license type.
#1641	Update the invite popover error message in the Admin Console to append guidance that points administrators to Signup Settings (Whitelist) for enabling user self-signup on Community Self-Hosted instances.	❌	The changes are limited to backend error definition and mapping. The PR does not modify the Admin Console UI, the invite popover, or any frontend text to include a link or guidance to Signup Settings/Whitelist.
#1641	Clarify UX messaging around workspace seat limit on Community Self-Hosted deployments so that administrators understand how to add team members without seeing billing-focused copy (e.g., 'Upgrade your license').	❌	While the backend error string is changed to a neutral 'Workspace member limit exceeded.' message, the issue’s requested UX improvement specifically concerns self-hosted guidance and navigation in the console. The PR does not implement that guidance or adjust the console error UX; it only changes a backend error message and mapping.

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[sourcery-ai]

Hey - I've left some high level feedback:

• Consider keeping the error message more generic and deriving deployment-specific guidance (e.g., Community Self-Hosted vs cloud) from configuration or a help link, so that this variant doesn’t hard-code environment-specific UI and product naming into a shared error type.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider keeping the error message more generic and deriving deployment-specific guidance (e.g., Community Self-Hosted vs cloud) from configuration or a help link, so that this variant doesn’t hard-code environment-specific UI and product naming into a shared error type.

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[tonicofonico]

@sourcery-ai review

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

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