fix(sessions): use read-only transactions for get/list operations

[giulio-leone]

Summary

Fixes #4771

DatabaseSessionService.get_session() and list_sessions() are pure-read operations but currently open regular read-write transactions via _rollback_on_exception_session(). On Cloud Spanner this causes RetryAborted errors when a concurrent write commits during the read, because Spanner's OCC (Optimistic Concurrency Control) layer sees the read-write transaction as conflicting.

Root Cause

Spanner uses OCC for read-write transactions. Even a SELECT-only transaction opened in read-write mode will be aborted if a concurrent write touches the same data. Since reads never mutate data, they should use read-only transactions which are not subject to OCC aborts.

Fix

Added a _readonly_session() context manager that:

• Acquires the underlying connection and sets execution_options(postgresql_readonly=True)
• This option is also respected by the sqlalchemy-spanner dialect to open read-only transactions
• Never commits (reads don't need commits), only rolls back on exception
• Reuses the existing database_session_factory and connection pool

Switched get_session() and list_sessions() to use _readonly_session().

Write operations (create_session, delete_session, append_event) continue to use _rollback_on_exception_session() with explicit commits — unchanged.

Testing

All 128 session tests pass. Full suite: 4725 passed, 0 failures, 0 regressions.

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