Handle missing source in sourcemap gracefully

[elias-ba]

Short Description

Use the returnNullOnMissing parameter of sourceContentFor to avoid throwing when the source file isn't in the sourcemap (e.g. when no adaptor is provided).

See #1249

Implementation Details

When running a job without an adaptor (openfn job.js), the compiler produces a sourcemap that doesn't contain the expected file entries. If the job then errors, sourcemap-errors.ts tries smc.sourceContentFor(fileName) which throws, triggering a confusing console.warn("Error occurred trying to resolve sourcemap for ...") before the actual error.

The fix passes true as the second argument to sourceContentFor, which makes it return null instead of throwing when the source isn't found. We then skip setting error.pos.src if there's no content.

QA Notes

1. Run openfn job.js without -a on a job that uses adaptor functions (e.g. get(...))
2. Verify the sourcemap warning no longer appears, only the actual ReferenceError is shown

AI Usage

• Code generation (copilot but not intellisense)
• Learning or fact checking
• Strategy / design
• Optimisation / refactoring
• Translation / spellchecking / doc gen
• Other
• I have not used AI

[elias-ba]

Hey @josephjclark the approach I am taking here was suggested by Claude Code and kind of made sense to me. But I want you to look at this and see if it's a fix you'd want. I am just learning here and exploring the code. Thanks man