Fix SourceEngine cache to refresh buffers after file edits

[JoshuaBatty]

Description

• Update SourceEngine::get_or_create_source_buffer to replace the cached buffer when the caller supplies different text.
• Fixes an LSP bug where diagnostics/token spans stayed stuck on the initial buffer, causing erroneous results after text edits.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[tritao]

Seems good, but I wonder if overall it would make sense to update the design in the future so we create a new source id every time we update the contents of the source buffer, to prevent spans pointing to outdated locations.