fix(typescript): fix null check for nullable query parameters and passthrough serialization

[jsklan]

Description

Fixes two issues in the TypeScript SDK generator:

1. TypeScript compilation errors when nullable query parameters (e.g., Date | null | undefined) are used in generated SDKs
2. Wire test failures where passthrough serialization was preserving both snake_case and camelCase keys (e.g., both version_number and versionNumber appearing in parsed results)

Link to Devin run: https://app.devin.ai/sessions/e8498bbef06245228a6e1502b58d2f36
Requested by: judah@buildwithfern.com (@jsklan)

Changes Made

Fix 1: Nullable Query Parameter Null Check

• Fixed the null check in GeneratedQueryParams.withQueryParameter() to use != null instead of !== undefined for nullable query parameters
• This ensures both null and undefined values are properly handled before calling methods like .toISOString() on Date parameters

Before (generated code):

if (fromDate !== undefined) {
    _queryParams.from_date = fromDate.toISOString(); // Error: fromDate could be null
}

After (generated code):

if (fromDate != null) {
    _queryParams.from_date = fromDate.toISOString(); // Safe: null and undefined are both excluded
}

Fix 2: Passthrough Serialization

• Fixed the passthrough() function in core-utilities to only preserve truly unknown keys
• Previously, spreading both raw and transformed objects caused duplicate keys with different casing
• Now filters out known raw/parsed keys before spreading, ensuring only genuinely unknown keys are preserved

Testing

• Verified fix by regenerating SDK from test fixture and confirming TypeScript build passes
• All 822 tests pass (including wire tests that were previously failing)
• Lint checks pass
• Unit tests added/updated

Human Review Checklist

• Verify nullable query parameter fix is correct for all nullable types (not just Date)
• Confirm passthrough fix doesn't break other serialization scenarios that rely on passthrough behavior
• Verify the type assertions (as Parsed & { [key: string]: unknown }) are safe and don't mask type errors
• Consider whether a seed test fixture should be added to prevent regressions

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