fix: use fullText setter — content from previous file no longer leaks

Stefan reported that opening a flow (hello) sometimes showed
content from a different flow (extract-with-approval). Cause:
flutter_code_editor's CodeController distinguishes between
`text` (the visible buffer, post-fold) and `fullText` (the
underlying source). Setting `_code.text = newFile` only
updates the visible buffer; the underlying `_code` data
structure retains the previous file's source, so subsequent
folding/scrolling can surface its content.

Switch all three load sites + the save site to the canonical
setter:

  Before  _code.text = text       (visible buffer only)
  After   _code.fullText = text   (replaces underlying source)

Save also moves to writing `_code.fullText` so any text the
operator collapsed behind a fold survives the round-trip.

Verified against flutter_code_editor 0.3.5 source — the
fullText setter calls `_updateCodeIfChanged` then re-emits
`TextEditingValue(text: _code.visibleText)`, which is the
documented "fully reset the editor" path.

Bump version 0.1.1 → 0.1.2. Studio's pubspec already pins
`ref: main` so the next `flutter pub upgrade` picks it up.

Signed-off-by: flemming-it <sf@flemming.it>

pubspec.yaml

@@ -1,6 +1,6 @@
 name: fai_studio_flow_editor
 description: Swappable inline YAML editor for F∆I Studio flows.
-version: 0.1.1
+version: 0.1.2
 publish_to: 'none'
 repository: https://git.flemming.ai/fai/studio-flow-editor