refactor(sandbox): centralize provider registration and configuration

[SaladDay]

Provider configuration, defaults and construction were distributed across Store, server setup and runtime-node paths, so adding an adapter required editing core vendor switches. Register Docker, microsandbox and E2B at one explicit boundary, with typed configuration normalization and shared preview/commit rules. Store retains encryption, transactions, generations and ownership; the five compute operations and optional native capabilities retain their existing semantics.

The registration also owns generated installer policies and node retention capacity. Database constraints validate structural configuration without enumerating provider names. Canonical E2B endpoints are persisted consistently and cleared on reset. The provider onboarding guide documents the boundary and generated contracts.

Validation:

• Focused configuration, generation, credential, reset and adapter tests passed against a dedicated PostgreSQL database.
• E2B SDK/helper tests (9 + 175) and microsandbox Go/Linux helper tests passed.
• Docker lifecycle and recovery-observation tests passed using isolated real Docker resources.
• Two fresh independent blind reviews completed; node-capacity and HTTP validation-error findings were fixed and covered by regression tests.
• Final-commit official-client CI passed, including standalone container distribution.
• Linux full backend/database checks passed; local Chrome console acceptance passed 85/86 with the single click-timeout case passing on isolated retry, and example browser acceptance passed 10/10.
• Final-commit GitHub make check passed: https://github.com/MiniMax-AI/parsar-core/actions/runs/36606467979
• Migration regressions verify official endpoint rollback, custom endpoint protection and immutable retained specifications.
• Integrated main through refactor(runtime): retire historical resource adoption #242, preserving removal of historical adoption paths.

This change does not qualify a live E2B cloud or microsandbox KVM lifecycle, or a live model workflow. Existing deployments and user data were not modified.