Fix tsconfig null override for extended configurations

[Copilot]

This PR fixes support for "nulling out" extended tsconfig.json properties by setting them to null.

Problem

When a tsconfig.json extends another tsconfig.json and tries to set a property to null to clear it, the current implementation doesn't handle this correctly. Consider:

// tsconfig-base.json
{
  "compilerOptions": {
    "types": ["node"]
  }
}

// tsconfig.json
{
  "compilerOptions": {
    "types": null
  },
  "extends": "./tsconfig-base.json"
}

Before this fix:

$ tsgo -p . --showConfig
{
  "types": ["node"],  // ❌ Still shows inherited value
  "configFilePath": "./tsconfig.json"
}

After this fix:

$ tsgo -p . --showConfig
{
  "configFilePath": "./tsconfig.json"
  // ✅ types field is omitted (successfully nulled out)
}

Root Cause

The issue was in the mergeCompilerOptions function which uses IsZero() to check if a field should be merged. In Go:

• null in JSON → Go nil slice → IsZero() returns true → merge skips it
• [] in JSON → Go empty slice → IsZero() returns false → merge applies it
• Not set → Go nil slice → IsZero() returns true → merge skips it

This meant the merge function couldn't distinguish between "not set" and "explicitly set to null".

Solution

• Added mergeCompilerOptionsWithRaw: A new merge function that's aware of explicitly set null values in the raw JSON
• Added handleExplicitNullValues: Processes raw JSON to detect fields explicitly set to null and applies them as overrides
• Updated tsconfig extending logic: Uses the new merge function during configuration extension

Test Coverage

Added comprehensive test cases:

• Single field null override (tsconfigNullOverride.ts)
• Multiple field null override (tsconfigNullOverrideMultipleFields.ts)
• Verified existing behavior still works (empty arrays, inheritance, etc.)

All existing tests pass, confirming backward compatibility.

Fixes #768.

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[jakebailey]

The tests added here are bad; delete them and instead add tests to the existing tsconfig parsing tests in the tsoptions package, which generate better baselines. Also, make sure this works for other compiler options, not just "types".

[Copilot]

The tests added here are bad; delete them and instead add tests to the existing tsconfig parsing tests in the tsoptions package, which generate better baselines. Also, make sure this works for other c...

Addressed in commit 6d21ec0. Deleted the bad compiler tests and added comprehensive tsoptions package tests that properly test null overrides for array fields (types, lib, typeRoots), string fields (outDir, baseUrl, rootDir), and mixed field types including Tristate fields. The tests use the baseline system and work with both JSON API and JSONSourceFile API.