fix(options): return explicit error on invalid JSON options #416

Problem

When users pass invalid JSON options to the Resvg constructor or renderAsync, the error is silently ignored and falls back to default options.

// Typo in option name - no error thrown
const resvg = new Resvg(svg, { dpii: 96 })

This makes debugging difficult as users don't know their options are being ignored.

Root Cause

In src/lib.rs:256-258 and src/lib.rs:391-393:

let js_options: JsOptions = options
    .and_then(|o| serde_json::from_str(o.as_str()).ok())
    .unwrap_or_default();

The .ok() call silently converts any serde error to None, then falls back to default.

Proposed Solution

Return explicit errors for:

• JSON syntax errors
• Unknown fields (already handled by deny_unknown_fields)
• Invalid field values (e.g., negative numbers for u32)
• Type mismatches

Error Examples

After fix:

• Unknown field: Invalid options: unknown field 'dpii', expected one of 'font', 'dpi', ...
• Invalid value: Invalid options: invalid value: integer '-1', expected u32 at line 1 column 25
• JSON syntax: Invalid options: expected ':' at line 1 column 10