Normalize subscriber fixture UUris to resource_id=0 for clarity

[PLeVasseur]

Context

In PR #77 review, a comment pointed out that subscriber UUris in test fixtures currently use non-zero resources (for example //authority-b/5678/1/1234).

For subscriber endpoint identity, using resource 0 is clearer and aligns with uProtocol endpoint-style addressing conventions.

Related spec clarification issue:

• usubscription: explicitly require SubscriberInfo.uri.resource_id = 0 for subscriber identity up-spec#321

Problem

Current fixture data can be read as if subscriber URI resource represents a topic/resource endpoint, which is confusing when the object is SubscriberInfo identity.

Proposed change

• Update subscription-related fixtures/tests to use subscriber UUris with resource_id = 0.
• Keep publisher/topic resource semantics unchanged (event topics remain in publish range).
• Update any related test assertions/docs to match the normalized fixture values.

Scope candidates

• up-streamer/src/routing/subscription_cache.rs
• up-streamer/src/routing/subscription_directory.rs
• up-streamer/src/routing/publish_resolution.rs
• up-streamer/src/data_plane/ingress_registry.rs
• Any other tests/examples using subscriber fixture URIs with non-zero resource IDs.

Why

This is a clarity/readability improvement that reduces ambiguity in tests and review discussions, and aligns local fixtures with the direction proposed in the upstream spec clarification.