feat(types): add __repr__ to content block and message types

[xodn348]

Closes #78

What changed

Added __repr__ methods to all content block and message types in src/claude_agent_sdk/types.py.

Types updated (17 total)

Content blocks: TextBlock, ThinkingBlock, ToolUseBlock, ToolResultBlock, ServerToolUseBlock, ServerToolResultBlock

Messages: UserMessage, AssistantMessage, SystemMessage, ResultMessage

Events: TaskStartedMessage, TaskProgressMessage, TaskNotificationMessage, MirrorErrorMessage, StreamEvent, RateLimitInfo, RateLimitEvent

Implementation details

• Added _truncate(s, max_len=100) module-level helper to bound long fields
• All __repr__ methods return concise single-line strings showing key fields
• Long string fields (text, thinking, error) are truncated at 100 chars
• Dict/list fields use _truncate(repr(field)) for safety
• UserMessage handles str | list[ContentBlock] union with isinstance check
• All methods have -> str return annotation (mypy strict compatible)

Tests

Added class TestRepr with 18 test methods in tests/test_types.py.
All 63 tests pass (pytest tests/test_types.py -v).