fix: enforce workspace path whitespace errors across create and runtime

[kaizhou-lab]

Summary

• split workspace-whitespace handling into dedicated create-time and runtime error codes with structured details
• enforce the create-time rule across conversations, teams, and cron APIs while keeping auto-provisioned temp workspaces valid when the user did not choose a path
• persist runtime send-failure tips with structured workspace error data so reloading history keeps the dedicated runtime code and workspace path

Verification

• cargo fmt --all
• cargo test -p aionui-common
• cargo test -p aionui-api-types
• cargo test -p aionui-api-types agent_error
• cargo test -p aionui-cron
• cargo test -p aionui-ai-agent send_error
• cargo test -p aionui-conversation send_message_rejects_legacy_workspace_with_runtime_error_code
• cargo test -p aionui-app --test message_e2e t2_1b_send_message_legacy_workspace_returns_runtime_whitespace_code
• cargo test -p aionui-app --test message_e2e t2_3b_warmup_legacy_workspace_returns_runtime_whitespace_code
• cargo test -p aionui-app --test cron_e2e cj3b_create_rejects_workspace_with_whitespace_segment
• cargo test -p aionui-app --test cron_e2e cj5b_run_now_legacy_workspace_uses_runtime_whitespace_code
• just push