Add to_json_value to WebAuthnIDLResponse, rename to_json → to_json_string

[Copilot]

Follow-up from PR #155 discussion: expose both serde_json::Value and formatted String outputs from the response trait.

• Added to_json_value(&self, ctx) -> Result<serde_json::Value, _> to WebAuthnIDLResponse
• Renamed to_json → to_json_string, now delegates to to_json_value internally
• JsonFormat retained on to_json_string for minified/pretty output

// Get a serde_json::Value for programmatic access
let value = assertion.to_json_value(&request)?;

// Get a formatted string (same as before, renamed)
let json = assertion.to_json_string(&request, JsonFormat::Prettified)?;

All call sites in examples and tests updated to to_json_string.

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[msirringhaus]

lgtm!