Add format-based negative test generation

[sinan-ozel]

What this PR does

Closes #— (no linked issue)

When an OpenAPI spec documents a 400 response for a POST or PUT endpoint, the
plugin now automatically generates one extra request body per required
string property annotated with a recognised format (email, uri, ipv4,
ipv6, hostname, idn-hostname, uuid). Each generated body replaces only
that one field with a deliberately invalid value while keeping the rest valid.

The server must respond 400 or 422. The plugin additionally requires the
response body to contain at least one non-whitespace string — an empty {}
body is treated as a contract violation.

Format-invalid objects are only generated for required properties and only
when 400 / 422 is documented in the spec's responses, so servers that do
not validate format are unaffected.

Changes

• src/pytest_openapi/case_generator.py — _INVALID_FORMAT_VALUES dict;
  generate_object_test_cases injects one format-invalid object per required
  format field; generate_string_test_cases appends invalid values when
  valid_only=False.
• src/pytest_openapi/contract.py — contains_invalid_format_value() and
  _has_nonempty_string_value() helpers; is_negative_test detection updated
  in test_post_endpoint_single, test_post_endpoint, and
  test_put_endpoint; 400/422 responses now require a non-empty body.
• src/pytest_openapi/__init__.py — bumped to v0.3.1.
• 8 new mock servers under tests/test_servers/ covering email, URI, UUID,
  IPv4, IPv6, hostname, a no-message-400 negative case, and URL port-range edge
  cases.
• tests/test_integration.py — 25 new integration tests.
• tests/test_unit.py — 4 new unit tests for the case generator.
• README.md — format-validation table, OpenAPI 3.0 vs 3.1 compatibility
  table.
• docs/how-it-works/test-cases.md — format-validation section.
• CHANGELOG.md — v0.3.1 entry.

Test plan

• All existing tests pass (docker compose -f tests/docker-compose.yaml …)
• New email format validation tests pass (valid address → 200, invalid → 400 with message)
• No-message 400 test correctly fails when server returns empty body
• URI, UUID, IPv4, IPv6, hostname format tests pass
• URL port edge cases: port 0 → 400, port > 65535 → 400, port 1–65535 → 200
• schema_based_api (no 400 documented, email field not required) unaffected — count stays at 13 passed

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