Add __str__ methods to content blocks and message types

[yuribodo]

Summary

Closes #78. Adds __str__ methods to user-facing content-block and message dataclasses so print(msg) produces readable output out of the box, removing the need for custom display_message helpers like the one in examples/streaming_mode.py.

Before

Users had to write their own isinstance ladder:

if isinstance(msg, UserMessage):
    for block in msg.content:
        if isinstance(block, TextBlock):
            print(f"User: {block.text}")
elif isinstance(msg, AssistantMessage):
    ...

After

print(msg)
# User: hi
# Claude: sure, [Tool use: Read] {'path': '/tmp/x'}
# [Task progress] Refactor tests (task_id=tsk_1, tokens=1200)
# [RateLimit: allowed_warning] five_hour @ 85%
# [Result: success] duration=1230ms, cost=$0.0015

Covered types

• Content blocks: TextBlock, ThinkingBlock, ToolUseBlock, ToolResultBlock
• Messages: UserMessage, AssistantMessage, SystemMessage, ResultMessage
• System-message subclasses: TaskStartedMessage, TaskProgressMessage, TaskNotificationMessage (override the parent __str__ so their description / status / summary isn't lost)
• Events: StreamEvent, RateLimitEvent

Hook-related types and MCP status types are TypedDict, not dataclasses, so they're out of scope here.

Design notes

• __str__ only — the dataclass-generated __repr__ is left alone. Python convention is str() for end users, repr() for developers, and keeping the default repr preserves useful debugging output in the REPL, logging, and pytest assertion messages. The issue title says "reprs" but the body clearly asks for print-friendly display, so __str__ felt like the conservative call. Happy to switch if maintainers prefer overriding __repr__ instead.
• Truncation: ToolUseBlock.input and ToolResultBlock.content are truncated to 200 chars (with ...) so a large Write/Edit payload doesn't blow up terminal output. The dataclass __repr__ still renders the full payload for debugging.
• Empty content: UserMessage / AssistantMessage with no content render as "User:" / "Claude:" — no trailing space.
• None content on ToolResultBlock: renders as "[Tool result]", not "[Tool result] None".

Test plan

• python -m pytest tests/ — 510 passed (26 new in TestMessageStr, covering truncation, empty/None content, mixed-block ordering, task subclasses, stream events, rate-limit events)
• python -m ruff check src/ tests/ — clean
• python -m ruff format --check src/ tests/ — clean
• python -m mypy src/ — no issues