Fixes ValidationException when using custom OpenAI-compatible providers that proxy to AWS Bedrock Converse API, or when using Bedrock directly in multi-turn conversations with empty tool results.

Problem

Bedrock Converse API strictly rejects empty text blocks with the error:
ValidationException: text content blocks must be non-empty

This occurred in two scenarios:

1. Custom providers (OpenAI format) proxying to Bedrock backend
2. Direct Bedrock API usage when tool calls return empty strings

The existing fix in bedrock_adapter.py only covered direct Bedrock connections. Custom providers using chat_completions API mode bypassed this sanitization, causing empty tool results to reach Bedrock and trigger validation errors.

Solution

Two-layer defense strategy:

Layer 1: Universal sanitization (run_agent.py)

Added content sanitization in _build_api_kwargs() before all API mode paths:

• Replaces None/empty/whitespace-only content with single space " "
• Handles both string content and multimodal content arrays
• Applies to ALL API modes: chat_completions, anthropic_messages, bedrock_converse, codex_responses, and provider profiles
• Benefits custom providers transparently

Layer 2: Bedrock-specific handling (agent/bedrock_adapter.py)

Enhanced two functions:

1. _convert_content_to_converse():

   • Strips whitespace from all text content
   • Filters out empty text blocks instead of keeping them
   • Returns space placeholder when all content is empty
   • Updated docstring to clarify placeholder behavior

2. Tool result handling in convert_messages_to_converse():

   • Explicit empty-string detection before creating tool result blocks
   • Uses space placeholder for empty/whitespace-only tool results
   • Preserves non-empty strings and JSON-serialized content

Technical Details

Placeholder choice: Single space " " is semantically neutral, minimally intrusive, and consistent with the existing pattern in anthropic_adapter.py (which uses "(no output)" for similar cases).

Compatibility:

• No impact on non-empty content
• Existing Bedrock adapter tests pass (30 tests)
• Existing Bedrock transport tests pass (100 tests)
• No conflict with existing bedrock_adapter.py safeguards

Testing

Validated against:

• Multi-turn conversations with empty tool results
• Custom provider (openapi-ait.ke.com) proxying to Bedrock
• Direct Bedrock Converse API usage
• Mixed content with empty and non-empty text blocks

Closes #9486

What does this PR do?

Related Issue

Fixes #

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

How to Test

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform:

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A