[Rust] Add persistent gRPC transport

[elenagaljak-db]

What changes are proposed in this pull request?

This PR implements the internal gRPC transport and recovery machinery for persistent streams. It deliberately does not expose a new public SDK type; that surface is added in #763.

The sender, receiver, connection, and supervisor tasks are shared between ephemeral and persistent streams through a typed transport seam. Persistent streams use the dedicated RPC while ephemeral behavior remains unchanged.

Persistent behavior includes:

• Create once, then resume the same stream_id after recovery.
• Stable logical offsets on the persistent wire protocol.
• New ingestion beginning at last_committed_offset + 1 after process-level resume.
• Landing-zone reconciliation for the lost-ack race where the server committed records before the client disconnected.
• Local completion of reconciled waiters and callbacks, with only offsets above the watermark resent.
• Existing backpressure, flush, close, callback, credential-refresh, and initial-retry behavior shared across transports.

The implementation remains behind eos. Its internal transport/recovery 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 check -p databricks-zerobus-ingest-sdk --features eos
• cargo test -p databricks-zerobus-ingest-sdk --features eos --lib — 156 tests passed
• End-to-end persistent coverage is added in [Rust] Test persistent stream resume #764.