feat: add custom repr to message and content block types

[javierdejesusda]

Summary

• Add concise, readable __repr__ methods to 14 dataclass types so
  print(message) produces useful output without helper functions
• Add _truncate private helper (80-char limit with ... suffix)
• 28 new tests covering all types + edge cases (quotes, newlines, backslashes)

Closes #78

Context

Issue #78 (filed by @blois) noted that the auto-generated dataclass repr
is too verbose for practical debugging. Example helpers like
display_message() in examples/streaming_mode.py exist solely because
the default output is unreadable.

Note: PR #326 by @martinritchie addressed the same issue but has been
pending review since Nov 2025. This PR takes a more focused approach
(repr only, no __str__, no version bump, no example changes) and uses
!r formatting for proper escaping of special characters.

Changes

src/claude_agent_sdk/types.py (+106 lines):

• _truncate(text, max_length=80) helper before content block section
• @dataclass(repr=False) + custom __repr__ on 14 types:
  • Content blocks: TextBlock, ThinkingBlock, ToolUseBlock, ToolResultBlock
  • Messages: UserMessage, AssistantMessage, SystemMessage, ResultMessage
  • System subtypes: TaskStartedMessage, TaskProgressMessage, TaskNotificationMessage
  • Events: StreamEvent, RateLimitEvent, RateLimitInfo

tests/test_types.py (+237 lines):

• TestTruncateHelper (5 tests)
• TestContentBlockRepr (10 tests incl. quotes/newlines/backslash edge cases)
• TestMessageRepr (4 tests)
• TestSystemMessageRepr (4 tests)
• TestResultAndEventRepr (5 tests)

Before / After

# Before (auto-generated, unreadable):
AssistantMessage(content=[TextBlock(text='Hello! I am Claude...a very long message'), ToolUseBlock(id='toolu_01X3mRqK5nKYrvSxnFEqqP1a', name='Read', input={'file_path': '/home/user/project/src/main.py', 'limit': 200, 'offset': 0})], model='claude-opus-4-6', parent_tool_use_id=None, error=None, usage={'input_tokens': 12345, 'output_tokens': 6789})

# After (concise, readable):
AssistantMessage(model='claude-opus-4-6', content=[TextBlock(text='Hello! I am Claude...a very long message'), ToolUseBlock(id='toolu_01X3mRqK5nKYrvSxnFEqqP1a', name='Read')])

Test plan

• python -m pytest tests/ — 355/355 pass
• python -m ruff check src/ tests/ — clean
• python -m ruff format src/ tests/ — clean
• python -m mypy src/ — only pre-existing errors in __init__.py
• Edge cases: quotes, newlines, backslashes properly escaped via !r
• No breaking changes — repr format is not a stable API