fix: catch tool errors in wrapToolCall middleware to prevent crash

[Himanshu Kumar (himanshu231204)]

Related Issue

Closes #426

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Problem

Any tool error during agent execution crashes the entire run via unhandled promise rejection in the langchain tool-call transformer.

Root Cause

When wrapToolCall middleware is present, tool errors get wrapped in MiddlewareError by the composition layer. ToolNode.#handleError then re-throws them as fatal because handleToolErrors !== true. The createToolCallTransformer rejects output promises that nobody catches, leading to UnhandledPromiseRejection and a process crash.

Crash Path

Tool throws error
  → wrapToolCall composition wraps in MiddlewareError
  → ToolNode.#handleError: errorFromMiddleware=true, handleToolErrors!==true
  → re-throws as fatal error
  → Agent run fails
  → createToolCallTransformer.fail() rejects output promises
  → No .catch() on promises
  → UnhandledPromiseRejection → CRASH

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Changes

src/agent/okf-middleware.ts

1. Restructured wrapToolCall — Catch tool execution errors inside the middleware and convert them to ToolMessages before they reach the composition layer. GraphInterrupt and AbortError are re-thrown as control flow.

2. Defensive addFrontmatterWarning — Wrapped validatePersistedFile in try/catch so frontmatter validation failures do not crash the run.

3. Added helper functions — isGraphInterruptLike(), isAborted(), toolErrorToMessage().

test/index-middleware.test.ts

5 new tests covering error resilience scenarios:

• Converts tool execution errors to ToolMessage instead of crashing
• Preserves non-Error throws as ToolMessage
• Re-throws GraphInterrupt errors without conversion
• Re-throws AbortError without conversion
• Swallows addFrontmatterWarning errors and returns original result

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Test Results

Suite	Result
index-middleware.test.ts	21 passed, 2 skipped (pre-existing mermaid timeout)
frontmatter-validator.test.ts	9 passed
New error resilience tests	5 passed

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Checklist

• Tests added/updated
• No breaking changes
• Error handling covers edge cases (GraphInterrupt, AbortError)
• Existing tests pass

[Himanshu Kumar (himanshu231204)]

Colin Francis (@colifran) Ready for review when you get a chance. Thanks!