Document infinite_sessions parity + Client::stop deferral (Bucket A.2/A.6)

Notes-only CHANGELOG entries closing out the remaining Bucket A items:

* A.2 (infinite_sessions) — already wired on both SessionConfig and
  ResumeSessionConfig as `Option<InfiniteSessionConfig>` with
  default-omit-on-the-wire semantics. Matches Node/Go (CLI-applied
  defaults when the field is absent). No code change; document the
  parity decision so it doesn't get re-flagged.
* A.6 (Client::stop error aggregation) — defer behind a Client-level
  session registry. Real aggregation requires iterating Session handles
  to disconnect-then-kill, but the Rust Client only tracks per-session
  channel senders today (router::SessionSenders), not Session instances.
  A cosmetic Result<(), Vec<Error>> change with no behavior shift would
  be pure pass-through wrapping. Tracked for Bucket B alongside the
  registry refactor.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: tclem

rust/CHANGELOG.md

@@ -161,3 +161,15 @@ public surface.
   so the concept doesn't apply. See `Client::from_streams` rustdoc.
 - `cargo semver-checks` runs in `continue-on-error` mode for 0.1.0; will
   flip to blocking once 0.1.0 is published and serves as the baseline.
+- `infinite_sessions: Option<InfiniteSessionConfig>` is wired on both
+  `SessionConfig` and `ResumeSessionConfig` and follows the same
+  default-omit-on-the-wire semantics as Node/Go: when `None`, the field
+  is skipped and the CLI applies its own default. No behavioral
+  divergence from the other SDKs.
+- `Client::stop` returns `Result<(), Error>` and currently kills the CLI
+  child process on the first failure rather than aggregating shutdown
+  errors across active sessions (Node returns `Error[]`). The Rust
+  `Client` does not yet hold strong/weak handles to its `Session`s — the
+  router only tracks per-session channel senders — so there is nothing
+  to iterate disconnect-then-kill across. Aggregation will land alongside
+  a Client-level session registry; tracked as Bucket B.