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>

pyproject.toml

@@ -31,7 +31,7 @@ keywords = ["intervals", "cycling", "running", "mcp", "ai"]
 "Upstream" = "https://github.com/mvilanova/intervals-mcp-server"
 
 [project.optional-dependencies]
-dev = ["pytest>=8.3.5", "mypy>=1.0.0", "ruff>=0.1.0", "pytest-asyncio>=0.21", "pre-commit", "hatch", "pytest-mock==3.12.0"]
+dev = ["pytest>=8.3.5", "mypy>=1.0.0", "ruff>=0.1.0", "pytest-asyncio>=0.21", "pre-commit", "hatch", "pytest-mock==3.12.0", "pytest-cov>=5.0", "cryptography>=42.0"]
 
 [tool.hatch.build]
 include = ["server.py", "utils/*.py", "README.md", ".env.example"]
@@ -114,7 +114,7 @@ default.unicode = true
 default.locale = "en-us"
 
 [tool.pytest.ini_options]
-addopts = "-q"
+addopts = "-q --cov=src/intervals_mcp_server --cov-report=term-missing --cov-fail-under=90"
 testpaths = ["tests"]
 python_files = "test_*.py"
 asyncio_default_fixture_loop_scope = "function"