fix(config): coerce env var strings to booleans and numbers in settings validation

[ixchio]

Hey team! Ran into this while debugging a config issue.

What's going on

When you use env vars in settings.json like this:

{
  "ui": {
    "autoThemeSwitching": "${GEMINI_AUTO_THEME:-true}"
  }
}

The resolved value is the string "true" — not boolean true. Zod blows up with a validation error because it expects an actual boolean. Same thing happens with number fields getting string values from env vars.

How I fixed it

Added z.preprocess coercion in both schema-building paths (buildPrimitiveSchema and buildZodSchemaFromJsonSchema). Pretty straightforward:

• "true" / "false" → true / false (case-insensitive, whitespace-trimmed)
• Numeric strings like "50" → 50
• Everything else (like "yes" or "maybe") still fails validation as it should

This avoids the schema-traversal complexity that tripped up earlier PRs — just hooks into Zod's own preprocessing pipeline.

Why not the existing PRs?

• fix(cli): coerce env-resolved boolean strings in settings #25608 doesn't handle $ref schema references and has type compatibility issues
• feat(config): support string coercion for boolean settings #25634 misses case-insensitive matching and whitespace trimming

Tests

Added 6 tests covering all the edge cases — coercion, case insensitivity, whitespace, rejection of non-boolean strings, and numeric coercion. All existing tests still pass.

Test Files  1 passed (1)
     Tests  31 passed (31)

Fixes #25573

cc @scidomino @NTaylorMullen would appreciate a look when you get a chance 🙏

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a configuration validation issue where environment variables injected into settings are treated as strings rather than their intended types. By integrating Zod's preprocessing pipeline, the system now correctly coerces these strings into booleans and numbers, preventing validation errors and improving the robustness of the configuration loading process.

Highlights

• Environment Variable Coercion: Implemented Zod preprocessing to automatically coerce string-based environment variables into their expected boolean or numeric types during settings validation.
• Robust Parsing: Added case-insensitive matching and whitespace trimming for boolean strings, ensuring reliable configuration parsing.
• Test Coverage: Added 6 new test cases to verify successful coercion of booleans and numbers, as well as the rejection of invalid input strings.

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[gemini-code-assist]

Code Review

This pull request implements string-to-boolean and string-to-number coercion for configuration settings validation. By utilizing Zod's preprocess feature along with new helper functions coerceBooleanString and coerceNumberString, the system now correctly handles environment variables that resolve as strings (e.g., "true", "50"). The changes include comprehensive unit tests covering case-insensitivity, whitespace trimming, and numeric conversion. I have no feedback to provide.