[Feature] Align Java and Python Tool outcome contracts

[joeyutong]

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Description

Background

Java and Python currently expose different Tool result contracts:

• Java Tool.call returns ToolResponse, which can explicitly represent success or failure without throwing an exception.
• Python Tool.call returns Any. An exception represents failure, while any normal return is treated as success because Python has no equivalent explicit error-result type.

Issue #925 and PR #926 introduce parallel Tool calls and a cross-language Outcome abstraction for durable batch execution. Outcome aligns invocation results as value or error, but it does not align the Tool-specific result contract: a successful Java Outcome<ToolResponse> may still contain an unsuccessful ToolResponse, while a successful Python Outcome contains an arbitrary value that is treated as a successful Tool result.

Discussion #901 and the corresponding Metrics PR #955 expose this difference through Tool success/failure metrics. The Metrics implementation follows each runtime's existing Tool contract and does not inspect arbitrary return payloads, so strict Java/Python outcome parity is not currently guaranteed.

Proposed direction

After PR #926, define one Tool outcome contract across Java and Python while preserving compatibility for existing Python Tools that return raw values:

• A normal raw Python return remains a successful Tool result.
• Both languages can explicitly return a successful or failed Tool result.
• An invocation exception remains a failed Tool result.
• Sequential and parallel Tool-call paths use the same mapping.
• Java/Python bridges preserve explicit Tool success, failure, and error details.
• ToolResponseEvent, Execution Events, and Tool/MCP metrics consume the same normalized outcome.
• MCP protocol-level error results should map to the same failed Tool outcome instead of being treated as successful content.

The runtime must not infer failure by inspecting arbitrary Tool payloads.

Expected validation

Cover native Java, native Python, and both cross-language directions for:

• normal success;
• explicit returned failure;
• thrown exception;
• sequential and parallel execution;
• MCP protocol-level failure.

Are you willing to submit a PR?

• I'm willing to submit a PR!