feat(dbapi): wire timeout parameter through Connection to execute_sql - #18484

…API Cursor.timeout (#18485)

Support the `timeout` execution option in the Spanner SQLAlchemy dialect
by wiring it directly to DBAPI `Cursor.timeout`.

### Changes
- **Statement-Level Timeout Isolation**: Pass `timeout` from
`execution_options` to `self.cursor.timeout` in
`SpannerExecutionContext.pre_exec()`. Scoping timeouts to the cursor
avoids mutating shared connection state and prevents statement timeouts
from leaking across queries on the same connection.
- **Connection Default Preservation**: Ensure `reset_connection()` does
not clear connection-level default timeouts when returning connections
to the pool.
- **Unit Tests**: Add tests verifying that `pre_exec` sets
`cursor.timeout` while leaving `connection.timeout` intact.
- **CI Dependency Compatibility**: Pin `sqlalchemy<2.1` and
`alembic<1.20` in `noxfile.py` and guard deprecation imports in
`test_suite_20.py` for CI stability.

### Companion PR
Depends on DBAPI `Cursor.timeout` support in #18484.

🤖 I have created a release *beep* *boop*
---


##
[3.72.0](google-cloud-spanner-v3.71.0...google-cloud-spanner-v3.72.0)
(2026-11-12)


### Features

* **dbapi:** wire timeout parameter through Connection to execute_sql
([#18484](#18484))
([98fc304](98fc304))
* declare Python3.15 support
([8775794](8775794))
* **spanner:** auth login support for Spanner Omni endpoints
([#18273](#18273))
([9e0548d](9e0548d))
* update API sources and regenerate
([#18797](#18797))
([be4d954](be4d954))


### Bug Fixes

* **google-cloud-spanner:** clear buffered DDL statements on rollback
and flush on autocommit
([#18636](#18636))
([15d6a2c](15d6a2c))
* **google-cloud-spanner:** do not replay DDL statements on transaction
retry
([#18625](#18625))
([78b9da2](78b9da2))
* **spanner:** correctly wrap async streaming calls in metrics
interceptor
([#18581](#18581))
([2d19b0a](2d19b0a))
* **spanner:** eliminate duplicate metrics capture and measure full
streaming latency
([#18605](#18605))
([a7678fd](a7678fd))
* **spanner:** record streaming operation completion when stream
finishes
([#18704](#18704))
([ddeae47](ddeae47))
* **spanner:** release transaction lock if inline begin fails
([#18409](#18409))
([dd24029](dd24029))
* **spanner:** stop deleting multiplexed sessions on rotation and close
([#18332](#18332))
([c38e960](c38e960))


### Performance Improvements

* **spanner:** add fast-path for multiplexed session acquisition
([#18317](#18317))
([9569b39](9569b39))
* **spanner:** avoid allocating empty RequestOptions on read and query
paths
([#18422](#18422))
([3743ff0](3743ff0))
* **spanner:** build ExecuteSqlRequest on the raw protobuf message
([#18379](#18379))
([6d91fec](6d91fec))
* **spanner:** eliminate redundant tracing span in run_in_transaction
([#18357](#18357))
([181e26a](181e26a))
* **spanner:** optimize built-in metrics hot path and harden concurrency
([#18329](#18329))
([f2a791f](f2a791f))
* **spanner:** optimize query option merging and prevent in-place
mutation
([#18358](#18358))
([8cb3c6a](8cb3c6a))
* **spanner:** optimize query parameter encoding with direct type
dispatch
([#18359](#18359))
([f06497c](f06497c))
* **spanner:** optimize query result decoding and type resolution
([#18330](#18330))
([ce72986](ce72986))
* **spanner:** optimize request ID header generation and retry closures
([#18408](#18408))
([ad03b95](ad03b95))
* **spanner:** optimize row popping in StreamedResultSet
([#18316](#18316))
([e66103d](e66103d))
* **spanner:** optimize single-chunk results and single-row lookups
([#18602](#18602))
([05b3775](05b3775))
* **spanner:** prune lock and begin event allocations on single-use
snapshots
([#18420](#18420))
([ab56473](ab56473))
* **spanner:** short-circuit trace_call when tracing is inactive
([#18356](#18356))
([58031be](58031be))
* **spanner:** support PartialResultSet.last with background stream
draining
([#18320](#18320))
([80efd63](80efd63))


### Documentation

* fix typos
([#18979](#18979))
([5c8baeb](5c8baeb))

---
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