test: raise coverage 64% -> 90% with behavior-focused tests + enforced gate

New suites assert real behavior, not just that code runs:
- test_types: workout serialization round-trips (recursive steps, camelCase
  keys, enum conversion) + __str__ formatting.
- test_api_client: request construction (URL/method/auth/body) and the full
  HTTP status-code -> message mapping.
- test_auth: RS256 JWT verification — valid -> AccessToken; expired/wrong-aud/
  wrong-issuer/wrong-key/missing-claim -> None; audience slash variants.
- test_server_setup: transport selection + start_server dispatch.
- test_events / test_activities / test_custom_items: request payloads
  (create vs update, POST/PUT/DELETE), delete accounting, JSON-content parsing,
  and error/empty branches.

Enforce >=90 via pytest --cov-fail-under=90; CI test job now gates the build.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/build.yaml

@@ -10,7 +10,24 @@ permissions:
   packages: write
 
 jobs:
+  test:
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    env:
+      API_KEY: test
+      ATHLETE_ID: i1
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+      - name: Install (with dev extras)
+        run: pip install --quiet ".[dev]"
+      - name: Test + coverage gate (>=90%)
+        run: pytest
+
   build:
+    needs: test
     runs-on: ubuntu-latest
     timeout-minutes: 20
     env: