Log a warning when removing a non-existent field

IndexSchema.removeField silently no-ops when the field doesn't exist:

// src/schema/schema.ts:278
removeField(fieldName: string): void {
    if (!Object.hasOwn(this.fields, fieldName)) {
        // TODO: log a warning
        return;
    }
    delete this.fields[fieldName];
}

Useful for diagnosing typos when callers expect a field to be present.

Acceptance criteria

• Warn when the field isn't present in the schema.
• Include the field name and the list of known field names in the warning.
• Unit test.

Read src/schema/schema.ts around IndexSchema.removeField at line 278, then find the existing unit tests for IndexSchema. Run those tests first and add coverage for removing a missing field; the work is done when the warning includes the requested field name and the known field names.