fix(proxy): [OCISDEV-845] skip space reconciliation on signed URL requests

[kobergj]

Summary

• Fix space management middleware removing users from all project spaces on signed URL (download) requests
• When no OIDC claims are in the request context (signed URL auth), the middleware now skips reconciliation entirely instead of interpreting absent claims as "user should have no access"
• Resolves intermittent download failures and transient "space not found" errors in EOSC environments

Technical Details

getSpaceAssignments() now returns nil (not empty map) when oidc.FromContext(ctx) is nil, and ServeHTTP skips processing when assignments are nil. This distinguishes "no claims available" from "claims say no access".

Test plan

• Deploy to EOSC test environment running claim-based space management
• Verify file downloads from project spaces succeed consistently
• Verify no more SpaceShared/SpaceUnshared oscillation in logs
• Verify OIDC-authenticated requests still correctly reconcile space memberships

Fixes: https://kiteworks.atlassian.net/browse/OCISDEV-845

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