Merge duplicate progress notifications during environment creation

[Copilot]

Problem

When creating a virtual environment with packages to install, users would see two separate, stacked progress notifications that were confusing and looked odd:

1. "Creating virtual environment named .venv using python version 3.13.0."
2. "Installing packages"

The second progress notification (displayed on top) represents just a different stage in the same creation operation, so displaying two separate notifications was unnecessary.

Solution

This PR merges the two progress notifications into a single, updating progress notification that leverages VS Code's progress API to update its message as the operation progresses through different stages.

Before:

• Two separate progress notifications
• Confusing UX with stacked notifications

After:

• Single progress notification that updates from "Creating virtual environment..." to "Installing packages..."
• Clean, streamlined user experience

Changes

API Enhancement

• Added suppressProgress?: boolean option to PackageManagementOptions type
• Allows package managers to skip showing their own progress notification when called as part of a larger operation with its own progress tracking

Core Implementation

• venvUtils.ts: Modified createWithProgress() to use the progress parameter to update the message to "Installing packages..." when package installation begins, and passes suppressProgress: true to api.managePackages()
• pipManager.ts: Updated manage() to check the suppressProgress flag and conditionally skip wrapping execution with withProgress() when the flag is true
• condaPackageManager.ts: Similar changes to respect the suppressProgress flag with proper handling of CancellationToken requirements

Testing

• Added comprehensive unit tests in venvProgressMerge.unit.test.ts to verify:
  • progress.report() is called with the "Installing packages..." message
  • suppressProgress: true is passed to managePackages()
  • Package management is skipped when no packages are specified
• All 197 unit tests passing with no regressions

Impact

• Improved user experience with cleaner, more intuitive progress notifications
• Fully backward compatible - new option is optional and defaults to false
• No breaking changes to existing package manager usage patterns
• Minimal, surgical changes to the codebase

Fixes #[issue_number]

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Merge the two into a single progress message</issue_title>
<issue_description>I think we should merge these two into single progress message.
Using VS Code API its possible to update the progress messages, hence displaying two isn't necessary.

Also looks weird to have two.
When the second progress (displayed on top) is just a different stage in the creation..
In which case we can use the progress API to update the progress message

</issue_description>

Comments on the Issue (you are @copilot in this section)

@eleanorjboyd great point yes! When creating an environment and installing packages as part of that flow, we should only have one progress which gets updated as the different steps are happening.

Fixes #925

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