Adds drain support to Spanner CDC source.

[acrites]

Before this change, we were calling the default implementation of truncateRestriction on drain, which, for bounded restrictions tries to finish evaluating the entire restriction. Both SDF's in the Spanner CDC source use bounded restrictions that are effectively unbounded. Overriding truncateRestriction to return null on these SDFs allows terminating immediately.

In addition, we had to change the cleanup action from DROP TABLE to DROP TABLE IF EXISTS to make deletion idempotent. Otherwise, if that work item gets retried by the backend, it will start fail-looping, preventing drain from completing.

We don't have a great way to test drain here, but I ran a test pipeline on Dataflow and verified that it can now drain successfully.

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[github-actions]

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[acrites]

The remaining failing PreCommit Spotless check says it's failing at :sdks:java:io:iceberg:checkstyleMain, but when I run that locally it passes.

[stankiewicz]

2026-09-04T08:36:38.6399796Z [ant:checkstyle] [ERROR] /runner/_work/beam/beam/sdks/java/io/iceberg/src/main/java/org/apache/beam/sdk/io/iceberg/cdc/sink/CdcRecord.java:20: You are using raw guava, please use vendored guava classes. [ForbidNonVendoredGuava]

#40015 forward fix under way