fix: prevent assistant-first tool-use history after context reduction

[KKamJi98]

Description

Motivation

SlidingWindowConversationManager.reduce_context() can produce invalid history for Gemini in long tool-heavy chats:

1. _truncate_tool_results() aborts early when latest toolResult is already truncated.
2. Fallback trimming can leave assistant toolUse as the first message.

Gemini rejects this with 400 INVALID_ARGUMENT (function call turn ...). This change keeps context reduction stable while preserving existing behavior for assistant text-only starts.

Resolves #1749

Public API Changes

No public API changes.

What changed

• Continue scanning older messages for truncatable toolResult entries when the latest candidate cannot be truncated.
• Skip already-truncated toolResult entries instead of returning early from _truncate_tool_results().
• Sanitize reduced history only when it starts with assistant toolUse (function-call start), preventing invalid Gemini turn order.
• Add regression tests for:
  • older candidate truncation,
  • assistant-first tool-use trimming behavior,
  • overflow when no valid user-start remains.

Related Issues

• [BUG] SlidingWindowConversationManager can emit assistant-first function call history causing Gemini 400 INVALID_ARGUMENT #1749
• Related context: [FEATURE] Improve tool result truncation strategy for graceful context reduction #1545

Documentation PR

N/A

Type of Change

Bug fix

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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