feat: add serverInstructions support to MCP server config

[roomote]

Related GitHub Issue

Closes: #11825

Description

This PR attempts to address Issue #11825 by adding support for user-defined serverInstructions in mcp_settings.json.

Changes:

1. Added serverInstructions to BaseConfigSchema (src/services/mcp/McpHub.ts): An optional serverInstructions: z.string().optional() field is now accepted in the config validation schema for all server types (stdio, sse, streamable-http).

2. Merged config instructions with protocol instructions (src/services/mcp/McpHub.ts - connectToServer): After connecting, user-defined serverInstructions from config are combined with protocol-provided instructions from client.getInstructions(). Config-defined instructions take precedence (appear first) when both exist.

3. Set instructions on placeholder connections (src/services/mcp/McpHub.ts - createPlaceholderConnection): The serverInstructions from config are visible even on disabled servers in the UI.

Config example:

{
  "mcpServers": {
    "my-server": {
      "type": "streamable-http",
      "url": "https://example.com/mcp",
      "serverInstructions": "Custom instructions for this server"
    }
  }
}

Feedback and guidance are welcome.

Test Procedure

• 7 new test cases added covering:
  • Schema validation for all server types (stdio, sse, streamable-http)
  • Optional field behavior (undefined when not provided)
  • Config-only instructions (no protocol instructions)
  • Combined config + protocol instructions
  • Placeholder connections for disabled servers
• All 54 tests pass: cd src && npx vitest run services/mcp/__tests__/McpHub.spec.ts

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue (see "Related GitHub Issue" above).
• Scope: My changes are focused on the linked issue (one major feature/fix per PR).
• Self-Review: I have performed a thorough self-review of my code.
• Testing: New and/or updated tests have been added to cover my changes.
• Documentation Impact: No documentation updates are required.
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Documentation Updates

• No documentation updates are required.

Additional Notes

This is a backward-compatible change -- the serverInstructions field is optional, so existing configs continue to work as before.

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