Refactor PythonResultResolver into modular components

[Copilot]

Decomposes the monolithic PythonResultResolver class into focused, testable components following the design spec in resultHandlerRefactor.md.

Changes

New Components

• TestItemIndex - Stateful per-workspace component managing ID mappings between Python test IDs and VS Code TestItems. Provides O(1) lookups with validation and tree-search fallback.
• TestDiscoveryHandler - Stateless handler for discovery payload processing and test tree population
• TestExecutionHandler - Stateless handler for execution results (pass/fail/skip/error/subtest)
• TestCoverageHandler - Stateless handler for coverage payload processing

Architecture

• PythonResultResolver becomes a pure facade delegating to static singleton handlers
• Only TestItemIndex maintains state (per workspace)
• Handlers are stateless and safely shared across workspaces

// Before: monolithic class with mixed concerns
class PythonResultResolver {
    public runIdToTestItem: Map<string, TestItem>;
    public _resolveDiscovery(...) { /* 50+ lines */ }
    public _resolveExecution(...) { /* 100+ lines */ }
    public _resolveCoverage(...) { /* 50+ lines */ }
}

// After: facade with delegated handlers
class PythonResultResolver {
    private testItemIndex: TestItemIndex;
    private static discoveryHandler = new TestDiscoveryHandler();
    private static executionHandler = new TestExecutionHandler();
    private static coverageHandler = new TestCoverageHandler();
    
    public _resolveDiscovery(payload, token) {
        PythonResultResolver.discoveryHandler.processDiscovery(
            payload, this.testController, this.testItemIndex, ...
        );
    }
}

Tests

• Added 16 unit tests for TestItemIndex covering registration, lookup, validation, and cleanup
• All 10 existing PythonResultResolver tests pass unchanged

Original prompt

execute the plan described in this markdown file

Created from VS Code via the GitHub Pull Request extension.

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