Add __repr__ to message and content block types

[kostakost2]

Fixes #78.

What this does

Adds human-readable __repr__ methods to all message types and content block dataclasses, so messages display usefully in REPLs, logs and debuggers without needing custom helpers.

Before:

>>> msg
AssistantMessage(content=[TextBlock(text='The capital of France is Paris...'), ToolUseBlock(id='toolu_01', name='bash', input={'command': 'ls'})], model='claude-sonnet-4-6', parent_tool_use_id=None, error=None, usage={...}, message_id='msg_01', stop_reason='end_turn', session_id='sess_01', uuid='uuid_01')

After:

>>> msg
AssistantMessage(model='claude-sonnet-4-6', content='The capital of France is Paris...', stop_reason='end_turn')

>>> TextBlock(text='Hello world')
TextBlock(text='Hello world')

>>> ToolUseBlock(id='toolu_01', name='bash', input={'command': 'ls -la'})
ToolUseBlock(name='bash', input_keys=['command'])

>>> ResultMessage(...)
ResultMessage(status='ok', turns=3, cost=$0.0042)

Changes

• TextBlock: truncated text preview (60 chars)
• ThinkingBlock: truncated thinking preview
• ToolUseBlock: tool name + input key names (values omitted — can be large/sensitive)
• ToolResultBlock: truncated content preview, error flag shown when true
• UserMessage: string content preview or block count
• AssistantMessage: model, first text block preview, stop_reason/error when set
• SystemMessage: subtype only
• ResultMessage: status (ok/error), turn count, cost when available

Tests

All 409 existing tests pass. The new reprs are purely additive — no behaviour changes.