Clarify fetch_cache_entry_with_wildcard semantics (effective wildcard merge + explicit no-match contract)

[PLeVasseur]

Context

fetch_cache_entry_with_wildcard in up-streamer/src/routing/subscription_cache.rs is intended to compute the effective subscriber lookup for one egress authority by merging:

• exact authority rows (entry)
• wildcard authority rows ("*")

(except when entry == "*", where wildcard should not be double-counted).

That merge behavior is correct and should be preserved.

Problem

The current return type Option<SubscriptionLookup> overloads None with two meanings:

• no matching subscribers
• cache access/lock failure

This makes the API ambiguous and harder to reason about.

Proposed Direction

• Represent no-match as an empty lookup.
• If failure signaling is needed, use an explicit Result<SubscriptionLookup, ...> contract:
  • Ok(empty_lookup) = no matching subscribers
  • Err(...) = lock/access failure
• Keep wildcard merge semantics unchanged (exact + wildcard).
• Update call sites/tests to align with the explicit contract.

Why

Current call sites already treat None as empty in practice, so this change aligns the API with real behavior and makes intent explicit.