style(pds-input, pds-checkbox): standardize helper and error message typography

[pixelflips]

Description

Standardizes helper and error message typography across form components to ensure visual consistency.

Changes:

• pds-input: Updated helper/error message font from --pine-typography-body-sm to --pine-typography-body-sm-medium
• pds-checkbox: Removed custom font-size override on error message (now inherits --pine-typography-body-sm-medium)
• pds-checkbox: Changed error message gap from --pine-dimension-050 to --pine-dimension-2xs

These components now match the typography used by pds-textarea, pds-select, and pds-switch.

Fixes DSS-49

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• unit tests
• e2e tests
• accessibility tests
• tested manually
• other:

Test Configuration:

• Pine versions:
• OS:
• Browsers:
• Screen readers:
• Misc:

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes
• Design has QA'ed and approved this PR

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