feat(validation): add context-aware postal code validators for UK, Canada, and Germany #512

Context

As outlined in the Contributor Roadmap (Level 2: Component Improvements), international tabular datasets frequently contain regional postal and zip codes. Adding dedicated validation support for common international formats helps users validate addresses and geographic fields safely without custom regex scripts.

Task

Implement context-aware postal code regex patterns in src/freshdata/fieldcheck.py for:

1. United Kingdom (UK): e.g. SW1A 1AA, EC1A 1BB, W1A 0AX
2. Canada: e.g. K1A 0B1, M5V 2T6
3. Germany (PLZ): e.g. 10115, 80331 (5-digit numeric strings)

Add corresponding unit test cases in tests/test_fieldcheck.py to ensure valid codes pass, invalid patterns fail cleanly, and edge cases (lowercase, varying whitespace) are properly handled or normalized.

Acceptance Criteria

• Add compiled regex patterns or helper functions in src/freshdata/fieldcheck.py supporting UK, Canadian, and German postal codes.
• Integrate into validate_fields pattern matching or field specification lookup.
• Add unit tests in tests/test_fieldcheck.py covering positive matches, negative matches, and edge cases.
• Tests execute fast and pass cleanly with pytest tests/test_fieldcheck.py --no-cov.
• Code adheres to style and type standards (ruff check . and mypy src/freshdata/fieldcheck.py).

Pointers

• Target validator module: src/freshdata/fieldcheck.py
• Test suite: tests/test_fieldcheck.py
• Contributor Roadmap: docs/community/contributor-roadmap.md

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