fix: resolve config fields by id fallback

[kocaemre]

Summary

• Resolve load_dict and load opfield lookups by field label first, then by field ID as a fallback.
• Preserve existing section label/ID matching behavior.
• Document the label-first, ID-fallback lookup behavior in USAGE.md.
• Add regression coverage for load_dict, load, and label-vs-ID collision precedence.

Why

opfield currently only matches field.label, so configurations cannot reference stable 1Password field IDs when labels differ or are user-facing. This adds the fallback requested in #50 while keeping label lookup precedence for backwards compatibility.

Fixes #50.

Test plan

• Added regression tests and confirmed RED failures before the implementation:
  • test_load_dict_resolves_field_by_id_when_label_differs
  • test_load_resolves_field_by_id_when_label_differs
• python3 -m pytest src/tests/test_config.py -q — 6 passed
• python3 -m pytest src/tests -q — 55 passed, 1 existing pytest warning
• python3 -m py_compile src/onepasswordconnectsdk/config.py src/tests/test_config.py
• git diff --check

[github-actions]

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