[Rust] Test persistent stream resume

[elenagaljak-db]

What changes are proposed in this pull request?

This PR adds stateful mock-server support and integration coverage for persistent streams.

The mock models durable state across connections:

• Create mints a unique stream_id.
• Acknowledged offsets remain stored by stream_id after disconnect.
• Resume returns the last committed offset.
• Unknown stream IDs return NOT_FOUND.
• Ingestion uses the persistent response envelope.

The lifecycle test creates a stream, queues multiple records, flushes once, closes, resumes at offset 1, ingests offset 2, flushes, and verifies the durable watermark. A second test covers an unknown stream ID. This infrastructure addition is recorded in rust/NEXT_CHANGELOG.md.

Stack

1. [Rust] Add persistent stream proto #761 — protobuf contract
2. [Rust] Add persistent gRPC transport #762 — gRPC transport and recovery engine
3. [Rust] Expose persistent stream API #763 — public Rust API
4. [Rust] Test persistent stream resume #764 — stateful mock and integration tests
5. [Rust] Document persistent streams #765 — documentation and runnable example

How is this tested?

• cargo test -p tests --test persistent_stream_tests — 2 tests passed
• cargo test -p databricks-zerobus-ingest-sdk --features eos --lib — 156 tests passed
• cargo fmt --all -- --check
• git diff --check