Allow completion providers for undefined shell types

[Copilot]

This PR enables completion providers to work with pseudoterminal-based terminals that don't have a defined shell type. Previously, these terminals were completely blocked from using custom completion providers due to an early return when shellType was undefined.

Problem

The issue was in terminalSuggestAddon.ts where completion requests would return early if shellType was undefined:

await this._shellTypeInit;
if (!this.shellType) {
    return; // This blocked pseudoterminal-based terminals
}

This prevented pseudoterminal-based terminals (like those created by extensions) from using any completion providers, even those that don't require a specific shell type.

Solution

1. Remove Early Return for Undefined Shell Type

• Kept the await this._shellTypeInit to wait for shell type initialization when possible
• Removed the early return that blocked completion providers when shellType is undefined
• Updated comments to clarify that completion providers can work with undefined shell types

2. Update Completion Service to Handle Undefined Shell Types

• Modified ITerminalCompletionService.provideCompletions() to accept TerminalShellType | undefined
• Updated provider filtering logic to allow providers when shellType is undefined:

  // Before: Would fail with undefined shellType
  if (provider.shellTypes && !provider.shellTypes.includes(shellType)) {
      return undefined;
  }
  
  // After: Allows providers to work with undefined shellType
  if (provider.shellTypes && shellType && !provider.shellTypes.includes(shellType)) {
      return undefined;
  }

3. Fix Telemetry Reporting

• Removed the requirement for shellType to be defined before reporting telemetry
• Updated getFirstShown() and updateShown() methods to handle undefined shell types gracefully
• Ensures completion latency is tracked regardless of shell type

Behavior Changes

Before: Pseudoterminal-based terminals (shellType = undefined) could not use any completion providers

After:

• ✅ Providers without shellTypes restrictions work with all terminals (including pseudoterminals)
• ✅ Providers with specific shellTypes work when shellType matches OR is undefined
• ✅ Telemetry is reported for all completions regardless of shellType
• ✅ All existing shell-specific filtering behavior is preserved

Testing

Added comprehensive tests to validate the provider filtering logic:

• Providers with no shellTypes restriction work with undefined shellType
• Providers with shellTypes restriction work with undefined shellType
• Providers with shellTypes restriction work with matching shellType
• Providers with shellTypes restriction are filtered out for non-matching shellType

This change enables extension-provided completion providers to work with pseudoterminal-based terminals while maintaining all existing functionality.

Fixes #253011.

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