Truncate left and right file contents for suggestions requests

[rtarcr]

Problem

When working on large files and left or right contents of the file are larger than 10240 characters, the request fails with a validation error.

Solution

Truncate the contents of the file to the left and right of the cursor

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[rahmaniaam]

Would be good to double check if the limitation is solely on character size rather than on byte size, since some characters might be 2 bytes instead of 1

[rtarcr]

Would be good to double check if the limitation is solely on character size rather than on byte size, since some characters might be 2 bytes instead of 1

Based on my testing and on VSCode implementation (https://github.com/aws/aws-toolkit-vscode/blob/master/src/codewhisperer/util/editorContext.ts#L28-L33), it is done on character basis