refactor(config): validated per-canton YAML for cantonal geoservice config

[monodo]

Summary

Replaces the ~1200-line hand-maintained Python dict (cantons_configuration/cantons.py) with one declarative YAML file per canton (cantons_configuration/data/<CODE>.yaml), validated by a hardened Pydantic schema that is enforced in CI so a bad config can never reach production.

Why the dict was a problem

• No validation — a typo (bad field name, missing URL, wrong harmonized value) failed silently at runtime against a live WMS instead of at load time.
• No type safety — consumed as raw dict end to end.
• Config mixed with code — editing cantonal data meant editing Python and redeploying; non-developers couldn't safely touch it.

What changed

• schema.py — typed CantonConfig / Layer / PropertyValue / HarmonyMapEntry models with extra='forbid' (so typos are rejected), reusing the existing GroundSuitability enum. Lenient exactly where the real data requires it: optional desc, layer-level target_harmonized_value (ZH zones) and ESRI id (GE/FR), empty-string URLs, and 3-to-5-element ground_control_point rows with mixed int/float/str.
• loader.py — loads and validates all YAML at import time (fail fast), aggregates every problem into one clear ConfigError, caches with lru_cache, and resolves paths package-relative so it works identically under uvicorn, pytest and AWS Lambda (Mangum). Validates on cold start, costs nothing on warm invocations.
• data/*.yaml — 26 per-canton files. cantons.py is now a thin shim re-exporting CANTONS from the loader, so every existing consumer keeps receiving the exact same legacy-shaped dict.
• Latent bug fixed — SO and BS carried name: 'NE' (copy-paste); corrected to match their code. The loader enforces name == filename going forward.
• Adds pyyaml runtime dependency (uv.lock updated).

Hardening / CI (prevents app-breaking config)

• tests/test_configuration_structure.py rewritten — 88 tests: every file loads + validates, every layer is usable, ESRI layers have an id, names match filenames, and the fail-fast paths (missing dir, empty dir, invalid YAML, unknown key, name/filename mismatch, bad GCP) all raise.
• New .github/workflows/validate_config.yml — on every push/PR: a fast standalone load_cantons() check plus the config test suite. A malformed YAML fails this check and blocks the merge/release before any deploy.

No API contract change

GET /v1/cantons, /v1/cantons/{code}, /v1/avalaible-cantons and the drill-category canton_config payload are unchanged — verified byte-for-byte JSON-equal between the old dict and the YAML-loaded output (modulo the intended SO/BS name fix).

Testing

• 131 tests pass (43 pre-existing + 88 new config tests); the full original suite is green.
• ruff check and ruff format --check clean.
• App and Lambda handler import cleanly; Dockerfile.lambda already copies src/drillapi so the YAML ships in the image.

Follow-ups (not in this PR)

• The legacy harmonyMap / loopLayers keys (LU only) are currently unused by any code path; preserved here for lossless migration. Worth a separate decision on whether to remove them.
• The NE/SO/BS entries remain inactive placeholder stubs with TEST layers; left as-is intentionally.