test: RPC-level coverage for Routing + UnixFS.add

[lthibault]

Context

PR #134 added the Routing capability (routing.capnp, src/rpc/routing.rs) and UnixFS.add. The HTTP client layer is covered by integration tests (tests/routing_integration.rs, tests/ipfs_integration.rs), but the Cap'n Proto RPC server layer is untested.

Gaps

• RoutingImpl RPC server — provide() and find_providers() in src/rpc/routing.rs are not exercised through Cap'n Proto serialization.
• ProviderSink streaming — the -> stream backpressure flow (build request, send, await, done) has zero test coverage. This is new Cap'n Proto syntax for us.
• EpochGuardedUnixFS::add — the add method wired in src/rpc/membrane.rs has no RPC-level test.

Approach

ipfs::HttpClient is a concrete struct — mocking requires either:

1. Refactoring behind a trait (allows unit tests with fake responses), or
2. Spinning up a lightweight HTTP test server (e.g. wiremock) that mimics Kubo endpoints.

Option 1 is cleaner long-term. Option 2 is quicker.

Acceptance

• RPC round-trip test for RoutingImpl::provide
• RPC round-trip test for RoutingImpl::find_providers exercising ProviderSink streaming
• RPC round-trip test for EpochGuardedUnixFS::add
• Epoch guard rejection test for routing methods