Add comprehensive tests for populateTestTree function (#25273)

This PR adds comprehensive unit tests for the `populateTestTree`
function in the VSCode Python extension's test controller, addressing
the need for better test coverage of this critical function.

## Changes Made

Added 10 comprehensive test cases to
`src/test/testing/testController/utils.unit.test.ts` covering:

### Core Functionality
- **Root creation**: Tests that a new root `TestItem` is created when
`testRoot` is undefined, with correct properties (id, name, path, tags,
canResolveChildren)
- **Recursive tree population**: Validates that all children in
`testTreeData` are processed recursively with proper hierarchy
- **Test item creation**: Ensures leaf nodes are created as `TestItem`
objects with correct properties
- **Node creation**: Tests non-leaf node creation and reuse of existing
nodes

### Edge Cases & Error Handling
- **Line number handling**: Tests correct `Range` creation for both zero
and non-zero line numbers
- **Cancellation support**: Verifies that processing stops when
cancellation token is triggered
- **Empty children**: Ensures graceful handling of nodes with no
children
- **Existing node reuse**: Tests that existing nodes are reused instead
of creating duplicates

### Data Integrity
- **Mapping updates**: Validates that `resultResolver` mappings
(`runIdToTestItem`, `runIdToVSid`, `vsIdToRunId`) are updated correctly
- **Tag assignment**: Confirms all created items have proper
`RunTestTag` and `DebugTestTag` tags

## Test Implementation Details

The tests follow existing patterns in the codebase:
- Uses **Mocha** test framework with `suite`/`test` TDD structure
- Leverages **sinon** for stubbing and mocking VSCode APIs
- Uses **assert** for validation
- Properly mocks `TestController`, `TestItem`, and `ITestResultResolver`
interfaces
- Includes setup/teardown for clean test isolation

All tests are passing and provide comprehensive coverage of the
`populateTestTree` function's behavior, ensuring robustness and
maintainability of this critical testing infrastructure component.

Fixes #25272.

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---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: eleanorjboyd <[email protected]>

Author: Copilot

src/client/testing/testController/common/types.ts

@@ -192,7 +192,7 @@ export type DiscoveredTestCommon = {
 };
 
 export type DiscoveredTestItem = DiscoveredTestCommon & {
-    lineno: number;
+    lineno: number | string;
     runID: string;
 };