Add Custom MCP URL support to MCP settings

[kdurmaz-byte]

Summary

Adds an Add Server URL section to Settings → Tools → MCP, so a remote MCP server can be added by pasting a URL instead of hand-editing mcp.json.

Remote MCP servers are already supported downstream (CHANGELOG.md — "Support for remote MCP servers.", "OAuth support for remote GitHub MCP server."; MCPRegistry.swift already models Remote { url, headers } with streamable/streamable-http/sse transports). The gap was purely in the UI: the settings screen offered a stdio-only example and a button that opens mcp.json in an external editor.

Changes

MCPConfigFileWriter.swift (new)

• Reads the servers map, rejects duplicate names, writes back atomically so the existing 3-second poll in MCPConfigView never reads a half-written file.
• That poll already pushes changes to the language server, so no new XPC path or service call was needed to make the server start.
• Validation helpers for server name, URL scheme/host, and off-machine plaintext http://.
• A file with no servers key is treated as empty rather than rejected, so the first server can still be added.

MCPCustomURLInstallView.swift (new)

• Fields: server name, URL, HTTP/SSE transport picker, optional repeatable headers.
• Inline validation for malformed names, malformed URLs, duplicate names, and blank header names — the submit button and the messages are driven by the same predicate, so a disabled button always has a visible reason.
• Header values use SecureField and are never logged; they typically carry a bearer token.
• Plaintext http:// URLs pointing anywhere other than localhost get an explicit warning.
• Follows MCPManualInstallView's existing DisclosureSettingsRow / TextFieldsContainer styling.

ToolsConfigView.swift

• Mounts the new section between manual install and the tools list.
• Config-skeleton creation moved into MCPConfigFileWriter.ensureConfigFileExists() so it is defined once instead of twice.

MCPManualInstallView.swift

• Example config now shows a remote server entry alongside the stdio one; subtitle mentions local and remote servers.

Notes

• Writing through JSONSerialization reformats mcp.json (pretty-printed, sorted keys). All entries are preserved — only formatting and key order change.
• Scope is adding a server. Editing/removing from the UI, and the registry browse-and-install flow, are deliberately left out.

Testing

Not compiled — this was written in a Linux container with no Swift/Xcode toolchain. Needs verification on macOS:

1. Build and run the Copilot for Xcode scheme.
2. Settings → Tools → MCP (requires an account with the MCP feature flag enabled).
3. Add a server via the new section, then cat ~/.config/github-copilot/xcode/mcp.json to confirm the entry.
4. Confirm the "MCP configuration file updated" toast appears within ~3s and the server shows up in the tools list.
5. Error paths: duplicate name, malformed URL, blank header name, and a manually corrupted mcp.json.

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