Don't delete RBI files for missing constants in LSP addon mode

[KaanOzkan]

Summary

When the LSP addon requests DSL generation for a constant that can't be found (e.g., due to a failed reload or a mid-edit syntax error), the existing code:

1. Deletes the RBI file for the missing constant
2. Raises an error that aborts DSL generation for all remaining constants
3. Purges "stale" files — any RBI that existed before but wasn't regenerated gets deleted

This is wrong in addon mode. The constant may be temporarily unavailable (user is mid-edit, reload failed, server has stale state). Deleting the RBI means Sorbet loses type information until the next successful generation.

Changes

Two deletion paths are fixed:

• abstract_dsl.rb (constantize): Skip deleting RBI and skip raising error when @lsp_addon is true
• dsl_generate.rb (execute): Skip purge_stale_dsl_rbi_files when @lsp_addon is true

Testing

End-to-end test with a real Rails runner client:

1. Generate valid RBI for NotifyUserJob
2. Hide the job file to simulate it becoming unloadable (e.g., syntax error after save)
3. Trigger reload + request DSL for the now-missing constant
4. RED (main): RBI deleted
5. GREEN (this branch): RBI preserved

All 4 existing addon tests pass.

Related

• Fixes https://github.com/Shopify/team-ruby-dx/issues/1711
• Part of https://github.com/Shopify/team-ruby-dx/issues/1762