refactor(network): consolidate proxy egress pipeline

[johntmyers]

Summary

Refactor sandbox proxy egress so CONNECT and forward-proxy traffic share explicit destination authorization and relay primitives while preserving the existing user-facing network behavior. This establishes the RFC #2155 foundation without introducing the later DNS or arbitrary TCP features, and keeps the shared path compatible with the middleware framework from #2027.

Related Issue

Related design: #2155
Related middleware implementation: #2027

Changes

• Introduce a shared egress pipeline with explicit destination intent, authorization, endpoint decisions, and relay preparation.
• Route CONNECT and non-CONNECT traffic through common HTTP, raw-stream, credential, and policy-generation boundaries.
• Preserve metadata-service handling, inference routing, credential injection, HTTP upgrades, raw relay, and current denial behavior.
• Pin relays to policy generations and close stale HTTP, upgraded, long-lived, and raw streams after policy activation changes.
• Reject overlapping endpoint selectors when connection or request-processing metadata is ambiguous.
• Configure rejected policy generation posture with [openshell.gateway] policy_validation_failure_mode in gateway.toml; default to fail_closed and allow the explicit retain_last_valid availability mode.
• Propagate the gateway TOML posture to sandbox supervisors as part of the effective config revision; individual sandboxes cannot override it.
• Emit OCSF configuration, finding, and denial rationale for validation failures and quarantine state.
• Add compatibility baselines, middleware coverage, ambiguity/recovery coverage, and shared-path E2E tests.
• Update gateway, Helm, architecture, and policy documentation for the new invariants and operator-visible failure posture.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)
• Live Docker gateway proxy E2E target: 10 passed
• cargo test -p openshell-core -p openshell-server -p openshell-sandbox
• mise run helm:lint
• mise run helm:test (64 passed)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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