fix: fail honestly without a provider

[EvanProgramming]

Summary

• raise a stable provider-unavailable error at the shared provider boundary
• return an actionable 503 REST error, one terminal SSE error, and a JSON-RPC MCP error
• cover REST, SSE, MCP, browser projection, and no-session-persistence regressions

Validation

• make lint
• make check
• KYROZEN_DB_PATH= make docs-check
• focused REST/SSE/MCP regression tests
• full core suite exercised; sandbox-only loopback and network/wheel dependency tests require CI permissions

Fixes #149