Allow callers to control prefix-cache partitioning

[h4x3rotab]

Problem

Private AI Gateway does not provide consistent caller control over prefix-cache partitioning.

For the active Kimi-K2.6 providers:

• Chutes forwards a caller's optional cache_salt to vLLM.
• Tinfoil strips cache_salt and derives its own value. It accepts the router-only user_cache_secret as the caller-controlled input, but the gateway does not currently translate the public field.

As a result, the same cache_salt request has different behavior across providers.

Desired behavior

Keep cache_salt caller-controlled across providers:

• Forward cache_salt unchanged to Chutes and direct vLLM providers.
• Translate cache_salt to user_cache_secret for Tinfoil, which derives the final salt.
• Do not derive, overwrite, or bind the partition to authenticated Redpill tenant identity.
• Preserve current provider behavior when the caller omits cache_salt.

Callers may choose distinct values for isolation or reuse a value to share a cache namespace. This is user-controlled partitioning, not a gateway-enforced tenant boundary.

Acceptance criteria

• A supplied cache_salt reaches Chutes and direct vLLM providers unchanged.
• The same public field reaches Tinfoil as user_cache_secret.
• Omitting cache_salt does not add a gateway-generated partition.
• Tests cover provider-specific request transformation and receipt consistency.
• Documentation explains the caller-controlled semantics and provider translation.

Evidence

Tinfoil's attested router strips cache_salt, consumes user_cache_secret, and derives the final salt: https://github.com/tinfoilsh/confidential-model-router/blob/v0.0.118/cache_salt.go