Improve error handling UX and move stream retry to BaseLLM (#11773)

* Improve error handling UX and move stream retry to BaseLLM

- Remove issue reporting buttons (GitHub/Discussions) from stream error dialog
- Add error detection for invalid API keys, OpenAI org verification,
  insufficient balance (402), and failed secret templating
- Update out-of-credits copy to match current messaging
- Add sk- prefix validation for OpenAI/Anthropic keys in add model form
- Update OpenRouter API key URL to workspaces/default/keys
- Move overloaded/malformed JSON retry from GUI streamThunkWrapper to
  BaseLLM _retryableStream to avoid glitchy UI from GUI-level retries
- Add overloaded/malformed JSON detection to core retry infrastructure

* fix: simplify retry architecture and make sk- validation a warning

- Remove _retryableStream from BaseLLM; overloaded/malformed JSON retry
  belongs in withExponentialBackoff at the HTTP fetch level
- Revert defaultShouldRetry/calculateDelay exports (no longer needed externally)
- Keep OpenRouter API key URL unchanged
- Make sk- prefix validation a warning instead of blocking error

* Fix stale comment referencing removed retry decorator

* Improve 402 Insufficient Balance error message clarity

* fix: improve copy - rename "View key" to "Check API key", use warning styles, simplify secret error message

* fix: use ternary for className to avoid space stripping by prettier

* style: fix prettier formatting in OutOfCreditsDialog

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Co-authored-by: continue[bot] <230936708+continue[bot]@users.noreply.github.com>