fix(conversation-manager): handle window_size=0 and reject negative values

[SuperMarioYL]

Problem

SlidingWindowConversationManager with window_size=0 silently fails to clear messages — the opposite of the intended behaviour documented in the TypeScript SDK.

Root cause in reduce_context:

trim_index = 2 if len(messages) <= self.window_size else len(messages) - self.window_size

With window_size=0 this sets trim_index = len(messages).
The while trim_index < len(messages) guard is immediately False, so the else branch fires:

• During routine management (e is None): logs a warning and returns — messages unchanged.
• During overflow recovery (e is not None): raises ContextWindowOverflowException — also wrong.

Negative window_size values weren't validated at construction time either.

Closes #2205.

Fix

1. __init__ — raise ValueError for window_size < 0 (parallel to the existing per_turn guard).
2. reduce_context — add a fast-path for window_size == 0: empty messages in-place, increment removed_message_count, and return. This matches the TypeScript SDK's explicit window_size=0 → "clear all messages" semantics (sdk-typescript#789).

Tests

Four new tests added to tests/strands/agent/test_conversation_manager.py:

Test	What it checks
test_window_size_negative_raises_value_error	window_size=-1 raises ValueError at construction
test_window_size_zero_clears_all_messages_on_apply_management	apply_management with window_size=0 empties messages and updates removed_message_count
test_window_size_zero_clears_all_messages_on_reduce_context	reduce_context with window_size=0 empties messages without overflow
test_window_size_zero_clears_on_overflow	reduce_context with window_size=0 still clears messages even when e is provided

All 44 existing + new tests pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[github-actions]

Issue: test_window_size_zero_clears_on_overflow is missing the removed_message_count assertion that the other two window_size=0 tests include. This means the test doesn't verify that the bookkeeping counter is updated in the overflow path.

Suggestion: Add assert manager.removed_message_count == 2 after assert messages == [] for consistency with the other tests and to fully verify the behavior.

[github-actions]

Assessment: Comment (leans Approve)

Clean, well-scoped fix that addresses a real bug with window_size=0 semantics and adds proper input validation. The PR description is excellent and the implementation is straightforward. One minor test gap noted inline — the overflow test is missing a removed_message_count assertion that the sibling tests include.