Add githubMcpServer.channel setting for MCP Insiders mode

[digitarald]

Adds a new github.copilot.chat.githubMcpServer.channel dropdown setting (stable / insiders) that sends an X-MCP-Insiders: true header to the GitHub MCP Server when set to insiders.

This enables access to experimental features in the GitHub MCP Server's Insiders mode.

Changes

• ConfigKey: Added GitHubMcpChannel (string, default 'stable', ConfigType.Simple)
• Setting schema: String enum with stable/insiders values, tagged experimental
• GitHubMcpDefinitionProvider: Config change listener, channel getter, X-MCP-Insiders header + |insiders version suffix
• NLS: Setting description with Learn more link, enum descriptions

Follows the existing pattern of readonly and lockdown settings.

[Copilot]

Pull request overview

Adds a new user-facing setting to choose the GitHub MCP Server “channel” (stable vs insiders) and wires that through to the MCP server definition so the extension can opt into GitHub MCP “Insiders mode”.

Changes:

• Introduces chat.githubMcpServer.channel configuration key (default stable).
• Updates GitHubMcpDefinitionProvider to react to channel changes and send X-MCP-Insiders: true (+ version suffix) when set to insiders.
• Adds configuration schema + NLS strings for the new dropdown setting.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/platform/configuration/common/configurationService.ts	Registers new GitHubMcpChannel config key and default value.
src/extension/githubMcp/common/githubMcpDefinitionProvider.ts	Applies channel to MCP headers/version and listens for configuration changes.
package.nls.json	Adds localized strings for the new setting and enum descriptions.
package.json	Adds the public settings schema entry (enum: stable / insiders, tagged experimental).

[digitarald]

@copilot fix merge conflict

[Copilot]

@copilot fix merge conflict

Resolved the merge conflict in package.json (the vscodeCommit hash) by taking the value from main. Commit: bfc7106