Add GPT- 5.5 tool experiment gates

[dileepyavan]

Summary

• Add experiment flags to control GPT-5.5 access to selected tools:
  • chat.gpt55GetChangedFilesTool.enabled gates get_changed_files
  • chat.gpt55ReadFileTool.enabled gates read_file
• Both flags are ExperimentBased booleans and default to true, preserving existing behavior unless an experiment disables them.
• Tool filtering uses isGpt55(endpoint) before tool-picker/explicit enablement logic, so GPT-5.4 is no longer affected by these gates.

Validation

• Editor diagnostics for touched files: no errors
• git diff --check passed
• npm run typecheck currently fails on unrelated Copilot CLI SDK type errors in src/extension/chatSessions/copilotcli/node/*

[Copilot]

Pull request overview

Adds experiment-based configuration gates in the Copilot extension to selectively disable specific tools when using GPT-5.4/GPT-5.5 model families, preserving current behavior by default while allowing experiments to turn off access.

Changes:

• Introduced two ExperimentBased boolean settings to gate get_changed_files and read_file for GPT-5.4/5.5.
• Updated tool enablement filtering to apply these gates before tool-picker and explicit enablement logic.

Show a summary per file

File	Description
extensions/copilot/src/platform/configuration/common/configurationService.ts	Adds new experiment-based config keys for GPT-5.4/5.5 tool gating.
extensions/copilot/src/extension/tools/vscode-node/toolsService.ts	Applies the new config gates during enabled-tools filtering for relevant model families.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 1