Fix RelaceEditTool to handle file not found errors gracefully

[codegen-sh]

Description

This PR fixes the error handling in the RelaceEditTool to properly handle file not found errors. Previously, when a file was not found, the tool would raise a FileNotFoundError exception, which would be propagated to the user as an error. This PR changes the behavior to return a RelaceEditObservation with an error status instead, which is consistent with how other errors are handled in the tool.

Changes

• Modified the relace_edit function in src/codegen/extensions/tools/relace_edit.py to return a RelaceEditObservation with an error status when a file is not found, instead of raising a FileNotFoundError.

Testing

This change ensures that when an invalid filepath is provided to the RelaceEditTool, the error is properly passed back to the assistant as part of the observation, rather than causing an exception.

Fixes the error:

FileNotFoundError('File not found: codegen_agi.py')

Now the error will be returned as part of the tool's observation, allowing the assistant to handle it gracefully.

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