Preserve MCP tool error outcomes

[xeophon]

Overview

Preserves MCP tool-declared outcome semantics across harness execution, typed messages, traces, replay, and provider conversion.

This PR is stacked on #2134, which owns the shared null/bash MCP client and its session lifecycle and replay-safety behavior.

Details

MCP CallToolResult contains both model-visible content and an isError outcome. The shared MCP client now returns both values, and the harness program carries the outcome into ToolMessage.is_error.

The typed outcome participates in graph identity and trace serialization, so success and failure results with identical content remain distinguishable after persistence and resume.

For chat-compatible providers, the localhost harness handoff uses a private marker that is removed before the provider request while adding a readable failure label to the model-visible content. Anthropic-native tool_result.is_error maps directly into the same typed representation.

Note

Preserve MCP tool error outcomes through the verifier pipeline

• Adds is_error field to ToolMessage in types.py and propagates it from call_mcp through the bash/null harness programs and into parsed messages.
• Introduces _vf_is_error as an internal wire flag in chat.py; when serializing for external consumption the flag is replaced with a "Tool execution failed:\n" content prefix, while internal paths (harness launch) preserve the machine-readable flag via include_internal=True.
• The Anthropic dialect in anthropic.py reads is_error from tool_result blocks, and message_hash in graph.py incorporates error vs. success into the digest so error and success tool messages hash differently.
• ChatDialect.apply_overrides rewrites messages containing _vf_is_error to error-prefixed content before forwarding upstream.
• Behavioral Change: tool messages that were previously indistinguishable from successful ones now carry a visible error prefix when sent to upstream models, and produce different cache keys.

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[xeophon]

Superseded by #2138, which carries the same head commit and is linked natively above #2134 in GitHub stack 2139. Closing this fork-head PR because GitHub native stacks require same-repository branches.