Add missing unit tests for FunctionEditPage

[Cragsmann]

Changes

• 🎁 Add 10 new unit tests for FunctionEditPage, bringing total coverage from 6 to 16 tests
• 🧹 Enhance CodeEditor mock to expose value, language, and emptyState props
• 🐛 Fix setupPushHandlers to use wildcard :sha param for getCommit (was hardcoded to head-sha, broke on cached commit SHAs)

New test cases

Edit page hook (useFunctionEditPage):

• File selection changes editor content
• Dirty tracking marks Save enabled after edit
• Dirty tracking resets (Save disabled) after save
• Edited content persists across file switches
• Language detection updates when selecting different file types
• Save triggers GitHub push API calls
• Save error shows danger alert
• Save button disabled while save is in progress

Toolbar integration:

• Error alert clears when next save succeeds

Empty/loading states:

• Empty state shows "Start editing" placeholder

Relates to SRVOCF-844

[dsimansk]

Great work, looks good to me. :)