ci: separate stable and prerelease

[wenytang-ms]

remove LLM tools and Exp feature for stable release.

To Preview bits

To RC bits

[Copilot]

Pull request overview

This PR separates stable vs prerelease (nightly/insiders) packaging so that LLM/Copilot-related contributions and the experimental setting are excluded from stable releases while remaining available in prerelease builds.

Changes:

• Gate Copilot LSP tool registration behind packageJSON.preview === true and default enableLspTools to false.
• Add a stable-manifest preparation script that strips languageModelTools/chat contributions and the experimental configuration property.
• Harden and update the nightly manifest generation (error handling, preview: true, prettier JSON output) and wire stable manifest generation into the RC pipeline.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/extension.ts	Adds prerelease gating for Copilot context tools and changes the config default for tool enablement.
scripts/prepare-stable-build.js	Generates a stable package manifest with LLM/chat contributions and the experimental setting removed.
scripts/prepare-nightly-build.js	Validates version parsing, ensures preview: true, and improves output formatting for the insiders manifest.
.azure-pipelines/rc.yml	Runs the stable-manifest preparation step before packaging the VSIX.
package.json	Bumps extension version.
package-lock.json	Updates lockfile version field to match the bumped version.

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