MCP OAuth 2.1, Slack thread improvements, and auth UX overhaul

Features

• MCP OAuth 2.1 and unified connection form: Added OAuth 2.1 support for MCP connections alongside a redesigned unified connection form. Credentials can now be assigned directly to agents from the connection UI.
• Curated MCP catalog: Introduced a structured MCP service catalog with pre-configured auth metadata, making it faster to connect to common services.
• Auth prefix dropdown with Basic base64 encoding: HTTP header rows now include an auth prefix dropdown that automatically base64-encodes user:password pairs for Basic auth.
• Slack get_thread with branch filtering: get_thread now returns the last N messages and supports filtering by from_external_id to scope results to a specific conversation branch.
• Synthetic assistant messages for handled Slack mentions: Already-handled Slack mentions now inject a synthetic assistant message into the thread context so agents have accurate conversation history.

Fixes

• Basic auth encoding is now always server-side: Basic auth base64 encoding happens at credential resolution time on the server for all credential sources, preventing double-encoding and client-side inconsistencies.
• Atlassian MCP auth corrected: Fixed Atlassian MCP connections to use Basic base64(email:token) format as required by the Rovo MCP spec.
• Slack DM replies no longer buried in stale sub-threads: Fixed an issue where DM replies were being routed into outdated sub-threads instead of the active conversation.
• Slack per-mention thread ID protected from stamp overwrite: The external thread ID set per-mention is no longer overwritten by later timestamp stamps.
• Slack trigger message seeding and false self-injection suppressed: Eliminated spurious self-injection and trigger message seeding that caused agents to process their own messages.
• MCP inline resource text extraction: Fixed extraction of inline resource.text content for all URI schemes, ensuring resource responses are correctly parsed.