Centralize tool result error handling in format with error_line helper

[LasmarKhalifa]

Part of #919

Foundation PR for the tool result formatter implementations.

[LasmarKhalifa]

• [PROTOTYPE] [WIP] Regex for parsing taskoutput results #984
• [PROTOTYPE] [WIP] REXML for parsing taskoutput tool results #983
• add taskoutput tool result formatter backed by nokogiri #960
• add task tool result formatter #963
• add agent tool result formatter #962
• add taskcreate tool result formatter #959
• add taskupdate tool result formatter #958
• add todowrite tool result formatter #957
• add skill tool result formatter #956
• add edit tool result formatter #955
• add write tool result formatter #954
• add grep tool result formatter #953
• add glob tool result formatter #952
• add read tool result formatter #951
• add bash tool result formatter #949
• Centralize tool result error handling in format with error_line helper #948 👈 (View in Graphite)
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[juniper-shopify]

Nice foundation PR! The error-handling centralization is clean — pulling the error path into an early-return at the top of format means every downstream formatter only needs to think about the success case. Makes the rest of your stack cleaner.

Two small things to address:

1. The error_line doc comment could use a couple of examples (see inline comment).
2. A test for error_line when content is nil would round out the edge-case coverage.

And lets switch to the xml parser we had talked about