Avoid unsupported PNG ScanJobs requests and validate eSCL formats (#8)

[boingball]

Issue

Addresses #8. The Epson XP-345 still needs real-device verification before the issue should be closed.

MintSCAN exposes a fixed JPEG/PNG/PDF selection even when the selected eSCL source advertises only JPEG/PDF. It also sent scan:DocumentFormatExt for the selected MIME whenever any extension was advertised, which could send a format the source never offered. A scanner can answer unsupported ScanJobs settings with HTTP 409 while still reporting Idle.

Changes

• Check the selected source's advertised pwg:DocumentFormat and scan:DocumentFormatExt values before a PNG job. If formats are advertised but PNG is absent, stop with a clear message rather than sending a predictable 409 or falsely producing a .png file.
• Emit scan:DocumentFormatExt only when the exact selected MIME is advertised by that source.
• Preserve JPEG/PDF and the behaviour of scanners with incomplete format capabilities; keep the existing compact XML and DPI/colour handling.
• Add host-side tests for JPEG/PDF-only capabilities, PNG in the extension list, absent/ambiguous declarations, whitespace and lookalike MIME strings. make check now includes these tests.

Verification

• The integration workflow and PR CI both completed successfully; make check passed.
• An actual scan against the reporter's Epson XP-345 remains outstanding. If the printer advertises PNG and still returns 409, please attach its ScannerCapabilities XML and the error response to identify any format/colour or extension interaction. This does not add software-side PNG transcoding for scanners without native PNG support.