DRIVERS-2782 Allow variable access to snapshotTimeout (#1870)

Author: prestonvasquez

source/sessions/snapshot-sessions.md

@@ -137,7 +137,8 @@ class ClientSession {
 }
 ```
 
-Getting the value of `snapshotTime` on a non-snapshot session MUST raise an error.
+The `snapshotTime` field MUST be read-only; in APIs that expose `snapshotTime` with a getter, attempting to read it on a
+non-snapshot session MUST raise an error.
 
 Transactions are not allowed with snapshot sessions. Calling `session.startTransaction(options)` on a snapshot session
 MUST raise an error.
@@ -270,6 +271,7 @@ C# driver will provide the reference implementation. The corresponding ticket is
 
 ## Changelog
 
+- 2025-12-17: Clarify snapshotTime semantics: the field is either read-only or validated.
 - 2025-09-23: Exposed snapshotTime to applications.
 - 2024-05-08: Migrated from reStructuredText to Markdown.
 - 2021-06-15: Initial version.

source/sessions/tests/README.md

@@ -270,10 +270,20 @@ Snapshot sessions tests require server of version 5.0 or higher and replica set
 
 Snapshot sessions tests require server of version 5.0 or higher and replica set or a sharded cluster deployment.
 
-- Start a session by calling `startSession` on with `snapshot = false`.
+- Start a session by calling `startSession` with `snapshot = false`.
+
+Drivers SHOULD implement one of the following approaches:
+
+**Approach 1: Validation (preferred if idiomatic)**
+
 - Try to access the session's snapshot time.
 - Assert that a client side error was raised.
 
+**Approach 2: Ensure Read-Only (if error-returning getters are not idiomatic)**
+
+- Attempt to mutate the session's `snapshotTime` field through any publicly accessible API.
+- Assert that the original session's `snapshotTime` remains unchanged.
+
 ## Changelog
 
 - 2025-09-25: Added tests for snapshotTime.