fix: update default JSignPdf runtime to 3.2.0

[vikas-kushwaha-dev]

Summary

Updates the default JSignPdf runtime from 3.1.0 to 3.2.0.

Closes #63.

Changes

• Updated the default JSignPdf version from 3.1.0 to 3.2.0.
• Added a regression test verifying that the generated default download URL points to the JSignPdf 3.2.0 minimal distribution.
• Updated the integration test for signing PDFs older than PDF 1.6, as JSignPdf 3.2.0 now successfully signs the existing fixture with the default parameters.
• Existing explicit JSignPdf path and download URL overrides remain unchanged.

Testing

The following checks pass locally:

• composer test:unit
• composer test:integration
• composer psalm
• composer cs:check
• git diff --check