fix: emit complete signal to callback handler when text block stops

[giulio-leone]

Issue

Closes #826

Problem

PrintingCallbackHandler checks kwargs.get('complete', False) to determine whether to print a trailing newline after text output, but no event in the streaming pipeline ever sets complete=True. This causes text output to never have proper line termination.

Root Cause

The streaming pipeline emits TextStreamEvent({'data': text, 'delta': delta}) for each text chunk but never signals completion. When contentBlockStop arrives, handle_content_block_stop() clears the accumulated text state—but yields no event to notify the callback handler that text streaming is done.

Meanwhile, EventLoopStopEvent and ModelStopReason both have is_callback_event = False, so they never reach the callback handler either.

Solution

Before calling handle_content_block_stop(), check if text was being accumulated (state['text'] is truthy). If so, yield a ModelStreamEvent({'complete': True}) after the stop handler runs.

This event:

• Has is_callback_event = True (non-empty dict → len(self.keys()) > 0)
• Does NOT trigger prepare() invocation_state merge (no 'delta' key in the dict)
• Is only emitted for text content blocks—not for toolUse or reasoningContent blocks

The fix is minimal (3 lines) and does not alter any existing event payloads or control flow.

Testing

• 4 new tests covering the complete signal behavior:
  • test_complete_event_emitted_for_text_block — verifies exactly 1 complete event after text stop
  • test_no_complete_event_for_tool_use_block — verifies no complete event for tool_use
  • test_no_complete_event_for_reasoning_block — verifies no complete event for reasoning
  • test_complete_event_with_multiple_text_blocks — verifies 1 complete event per text block
• All 1870 existing tests pass with updated expected event lists
• Updated tests: test_process_stream, test_stream_messages, test_agent__call__callback, test_stream_e2e_*

Changes

File	Change
src/strands/event_loop/streaming.py	+3 lines: check had_text, yield ModelStreamEvent({'complete': True})
tests/strands/event_loop/test_streaming.py	+124 lines: 4 new tests + updated expected events
tests/strands/agent/test_agent.py	+1 line: updated callback expectations
tests/strands/agent/hooks/test_agent_events.py	+6 lines: updated E2E expected events

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[giulio-leone]

Friendly ping — emits the complete signal to the callback handler when a text content block stops, fixing missing completion callbacks.