6. Add playground connection feature

[anish-sahoo]

• playground target connection controls, setup logs, schema discovery, and health polling
• includes colocated component/hook unit tests
• cog-playground stack: 1. Add Go playground server and handlers #3109, 2. Add playground frontend scaffold and early CI #3108, 3. Add core playground types and utilities #3114, 4. Add shared playground UI and JSON editor #3110/5. Add Cog playground API client #3106, 6. Add playground connection feature #3105/7. Add playground input controls and validation #3111/8. Add playground prediction controls and output #3107, 9. Wire up Cog playground and documentation #3113, 10. Compose playground app and add browser e2e #3112

[ask-bonk]

LGTM

github run

[anish-sahoo]

note: ci will pass after pr 5 (#3106) is merged

[ask-bonk]

LGTM

github run