feat(errors): add NotFoundError with resource_type and resource_id

[xJeffx23]

Description

Implements NotFoundError, a new exception raised on HTTP 404 responses. Exposes resource_type and resource_id attributes populated from the response body JSON, so callers can distinguish a missing payment from a
missing invoice without parsing the message string.

Fixes #(issue)

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Unit tests added in tests/test_errors.py covering:

• NotFoundError is instance of ShadeError
• resource_type and resource_id parsed from JSON response body
• Explicit kwargs override body values
• Graceful None when body is missing, lacks fields, or is invalid JSON
• from_response factory sets status_code=404 automatically
• Package root exports NotFoundError

Run locally:
pytest tests/test_errors.py -v # 11 passed

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own 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 unit tests pass locally with my changes

Closes #18

[codebestia]

LGTM!
Nice Implementation.
Thank you for your contribution.