[pdfx] Align default image format with native implementation (JPEG → PNG)

[ktanaka101]

Description

What & Why

Both the public docs and the native Android / iOS layers say
PdfPage.render() defaults to PNG when format is omitted.
The Dart signature, however, still defaulted to PdfPageImageFormat.jpeg.

// produces PNG bytes even though the API claims JPEG
final img = await page.render(format: PdfPageImageFormat.jpeg);

That mismatch triggers a false-positive from the
avoid_redundant_argument_values
linter: it tells you to drop the argument, but doing so would silently
change the result from JPEG → PNG.

This PR aligns the Dart default with reality:

- PdfPageImageFormat format = PdfPageImageFormat.jpeg,
+ PdfPageImageFormat format = PdfPageImageFormat.png,

Tests

Added test/pdfx_test.dart.

State	Result	Meaning
Pre-patch	✅ Pass	Reveals that PNG was already returned at runtime.
Post-patch	✅ Pass	API, docs and native behaviour are now consistent.

Impact

• No breaking change – callers already received PNG bytes by default.
• Eliminates the misleading linter warning, improving DX.

Thanks for reviewing!