refactor(linter): remove `source_text` field from `ContextSubHost` (#26079)

`source_text` field was added to `ContextSubHost` in #25280 to work around the code that runs JS plugins (`run_external_rules`) nuking the whole of `Semantic`, replacing `semantic.source_text` with an empty string.

#26077 has removed that behavior, so we can now return to using `semantic.source_text`, and it remains what it should be, even after `run_external_rules`.