Add raises assertion

[JPDuchesne]

Summary

• Add Spock-style raises exception conditions for Then blocks: raises ExceptionClass wraps the preceding When block in an exception assertion, and e = raises ExceptionClass captures the exception for further property assertions.
• Rename interaction outcome nodes from rspock_returns / rspock_raises to rspock_stub_returns / rspock_stub_raises to free up the clean rspock_raises type for exception conditions.
• Add documentation (README section + CHANGELOG entry) and comprehensive test coverage including data-driven Where block integration.

Test plan

• Unit tests for StatementParser covering raises detection (direct, assigned, and negative cases)
• Unit tests for ThenBlock validation (at most one raises per block)
• Unit tests for ExpectBlock validation (raises disallowed)
• Interaction parser tests updated for renamed stub_* node types
• Integration tests in example_rspock_test.rb: basic raises, capture with property assertion, and data-driven raises with Where block
• All existing tests pass (no regressions from node rename)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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