[Backend] backend/src/config/sandbox.ts hardcodes sandbox mode defaults with no schema validation of the SANDBOX_MODE env var

[grantfox-oss]

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Why this matters

backend/src/config/sandbox.ts reads sandbox-related environment variables to decide default behavior. If SANDBOX_MODE (or similar) is set to an unexpected string like "TRUE" or "yes" instead of the expected value, silent truthy/falsy coercion could enable or disable sandbox mode unintentionally in production, which is a meaningful safety toggle given sandboxMiddleware trusts it.

Acceptance criteria

• Add explicit, documented accepted values for sandbox-related env vars
• Fail fast at startup with a clear error on an unrecognized value rather than silently coercing
• Add a unit test for the parsing logic

Files to touch

• backend/src/config/sandbox.ts

Out of scope

• Changing sandbox middleware request-time behavior (tracked separately)
• Adding new sandbox modes

[Michvista]

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[Oyinkans0la12]

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[pixels26]

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[abdulquadri123]

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[Ajibose]

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